Saxon

When retrieving data from a table having a column with null value with the following code

   <xsl:template name="xsl:initial-template" match="/">
      <xsl:variable name="row" select="sql:query($connection, $table, '*', $condition)"/>
      <xsl:sequence select="shared:log('DEBUG', concat('Nb of rows = ', count($row)))"/>
      <xsl:variable name="output-uri" select="shared:path-to-uri($output-file)"/>
      <xsl:sequence select="shared:log('DEBUG', concat('Exporting ', $output-uri))"/>
      <xsl:result-document href="{$output-uri}" method="xml">
         <xsl:element name="{$table-synonym}s">
            <xsl:for-each select="$row">
               <xsl:variable name="row-map" select="."/>
               <xsl:element name="{$table-synonym}">
                  <xsl:for-each select="map:keys($row-map)">
                     <xsl:attribute name="{lower-case(.)}" select="map:get($row-map, .)"/>
                  </xsl:for-each>
               </xsl:element>
            </xsl:for-each>
         </xsl:element>
      </xsl:result-document>
   </xsl:template>

we get the following error message:

java.lang.NullPointerException
	at java.util.Objects.requireNonNull(Objects.java:203)
	at net.sf.saxon.str.StringView.<init>(StringView.java:26)
	at net.sf.saxon.str.StringView.of(StringView.java:31)
	at net.sf.saxon.value.StringValue.<init>(StringValue.java:106)
	at net.sf.saxon.value.StringValue.<init>(StringValue.java:91)
	at com.saxonica.functions.sql.SQLFunctionSet.sqlQueryResult(SQLFunctionSet.java:64)
	at com.saxonica.functions.sql.SQLQueryFn.call(SQLQueryFn.java:84)
	at net.sf.saxon.expr.SystemFunctionCall$SystemFunctionCallElaborator.lambda$elaborateForPull$3(SystemFunctionCall.java:625)
	at net.sf.saxon.expr.elab.EagerPullEvaluator.evaluate(EagerPullEvaluator.java:37)
	at net.sf.saxon.expr.LetExpression$LetExprElaborator.lambda$setAllVariables$0(LetExpression.java:839)
	at net.sf.saxon.expr.LetExpression$LetExprElaborator.lambda$elaborateForPush$6(LetExpression.java:940)
	at net.sf.saxon.expr.instruct.TemplateRule.applyLeavingTail(TemplateRule.java:393)
	at net.sf.saxon.trans.Mode.handleRuleNotNull(Mode.java:587)
	at net.sf.saxon.trans.Mode.applyTemplates(Mode.java:521)
	at net.sf.saxon.trans.XsltController.applyTemplates(XsltController.java:684)
	at com.saxonica.functions.extfn.TransformFn.call(TransformFn.java:103)
	at net.sf.saxon.expr.SystemFunctionCall$SystemFunctionCallElaborator.lambda$elaborateForPull$3(SystemFunctionCall.java:625)
	at net.sf.saxon.expr.elab.LazyPullEvaluator.evaluate(LazyPullEvaluator.java:39)
	at net.sf.saxon.expr.SystemFunctionCall$SystemFunctionCallElaborator.lambda$elaborateForPull$2(SystemFunctionCall.java:615)
	at net.sf.saxon.expr.elab.LazyPullEvaluator.evaluate(LazyPullEvaluator.java:39)
	at net.sf.saxon.expr.SystemFunctionCall$SystemFunctionCallElaborator.lambda$elaborateForPull$2(SystemFunctionCall.java:616)
	at net.sf.saxon.expr.LetExpression$LetExprElaborator.lambda$elaborateForPull$5(LetExpression.java:927)
	at net.sf.saxon.expr.instruct.Block$BlockElaborator$BlockIterator.getNthChildIterator(Block.java:832)
	at net.sf.saxon.expr.instruct.AbstractBlockIterator.next(AbstractBlockIterator.java:66)
	at net.sf.saxon.value.SingletonClosure.asItem(SingletonClosure.java:111)
	at net.sf.saxon.value.SingletonClosure.iterate(SingletonClosure.java:71)
	at net.sf.saxon.expr.UserFunctionCall$UserFunctionCallElaborator.lambda$elaborateForPull$0(UserFunctionCall.java:746)
	at net.sf.saxon.value.SingletonClosure.asItem(SingletonClosure.java:111)
	at net.sf.saxon.value.SingletonClosure.materialize(SingletonClosure.java:162)
	at net.sf.saxon.expr.elab.LocalVariableEvaluator.evaluate(LocalVariableEvaluator.java:31)
	at net.sf.saxon.expr.SystemFunctionCall$SystemFunctionCallElaborator.lambda$elaborateForPull$1(SystemFunctionCall.java:605)
	at net.sf.saxon.expr.sort.SortExpression$SortExprElaborator.lambda$elaborateForPull$0(SortExpression.java:511)
	at net.sf.saxon.expr.instruct.ForEach$ForEachElaborator.lambda$elaborateForPush$5(ForEach.java:737)
	at net.sf.saxon.expr.instruct.Block$BlockElaborator.lambda$elaborateForPush$1(Block.java:853)
	at net.sf.saxon.expr.instruct.NamedTemplate.expand(NamedTemplate.java:269)
	at net.sf.saxon.expr.instruct.CallTemplate$CallTemplateElaborator.lambda$elaborateForPush$1(CallTemplate.java:633)
	at net.sf.saxon.expr.instruct.Block$BlockElaborator.lambda$elaborateForPush$2(Block.java:864)
	at net.sf.saxon.expr.LetExpression$LetExprElaborator.lambda$elaborateForPush$6(LetExpression.java:942)
	at net.sf.saxon.expr.instruct.Block$BlockElaborator.lambda$elaborateForPush$1(Block.java:853)
	at net.sf.saxon.expr.LetExpression$LetExprElaborator.lambda$elaborateForPush$6(LetExpression.java:942)
	at net.sf.saxon.expr.instruct.NamedTemplate.expand(NamedTemplate.java:269)
	at net.sf.saxon.expr.instruct.CallTemplate$CallTemplateElaborator.lambda$elaborateForPush$1(CallTemplate.java:633)
	at net.sf.saxon.expr.LetExpression$LetExprElaborator.lambda$elaborateForPush$6(LetExpression.java:942)
	at net.sf.saxon.expr.instruct.ForEach$ForEachElaborator.lambda$elaborateForPush$5(ForEach.java:741)
	at net.sf.saxon.expr.LetExpression$LetExprElaborator.lambda$elaborateForPush$6(LetExpression.java:942)
	at net.sf.saxon.expr.instruct.NamedTemplate.expand(NamedTemplate.java:269)
	at net.sf.saxon.expr.instruct.CallTemplate$CallTemplateElaborator.lambda$elaborateForPush$1(CallTemplate.java:633)
	at net.sf.saxon.expr.instruct.Block$BlockElaborator.lambda$elaborateForPush$4(Block.java:895)
	at net.sf.saxon.expr.LetExpression$LetExprElaborator.lambda$elaborateForPush$6(LetExpression.java:942)
	at net.sf.saxon.expr.TryCatch$TryCatchElaborator.lambda$elaborateForPush$1(TryCatch.java:340)
	at net.sf.saxon.expr.instruct.NamedTemplate.expand(NamedTemplate.java:269)
	at net.sf.saxon.trans.XsltController.callTemplate(XsltController.java:872)
	at net.sf.saxon.s9api.Xslt30Transformer.callTemplate(Xslt30Transformer.java:503)
	at net.sf.saxon.Transform.processFile(Transform.java:1375)
	at net.sf.saxon.Transform.doTransform(Transform.java:879)
	at net.sf.saxon.Transform.main(Transform.java:83)

Retrieving data with the instruction sql:query works fine:

   <xsl:template name="xsl:initial-template" match="/">
      <xsl:variable name="row" as="element()*">
         <sql:query connection="$connection" table="{$table}" where="{$condition}" column="*"/>
      </xsl:variable>
      <xsl:sequence select="shared:log('DEBUG', concat('Nb of rows = ', count($row)))"/>
      <xsl:variable name="output-uri" select="shared:path-to-uri($output-file)"/>
      <xsl:sequence select="shared:log('DEBUG', concat('Exporting ', $output-uri))"/>
      <xsl:result-document href="{$output-uri}" method="xml">
         <xsl:element name="{$table-synonym}s">
            <xsl:for-each select="$row">
               <xsl:element name="{$table-synonym}">
                  <xsl:for-each select="*">
                     <xsl:attribute name="{lower-case(local-name(.))}" select="."/>
                  </xsl:for-each>
               </xsl:element>
            </xsl:for-each>
         </xsl:element>
      </xsl:result-document>
   </xsl:template>