After JDK-8062518, the following test code will throw NPE.

TestTrans.java
import java.io.File;
import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerFactory;
import javax.xml.transform.stream.StreamResult;
import javax.xml.transform.stream.StreamSource;

public class TestTrans {
    public static void main(String[] args) throws Exception {
        Transformer transformer = TransformerFactory.newInstance().newTransformer(new StreamSource(new File("librarySet01.xsl")));

        transformer.transform(new StreamSource(new File("librarySet01.xml")), new StreamResult("test1.out"));
    }
}


librarySet01.xsl :
<?xml version="1.0"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0"
    xmlns:set="http://exslt.org/sets"
    exclude-result-prefixes="set">
<xsl:output method="xml" encoding="UTF-8" indent="yes" />

<xsl:template match="/">
  <out>
    <test desc="the second should be empty, but not null">
      <xsl:copy-of select="set:leading(/doc/*, /doc/non-existing)"/>
    </test>
  </out>
</xsl:template>

</xsl:stylesheet>


librarySet01.xml:
<?xml version="1.0" encoding="UTF-8"?>
<doc>
  <num>1</num>
  <str>a</str>
  <num>2</num>
</doc>


Program output:
java TestTrans
ERROR:  ''
Exception in thread "main" javax.xml.transform.TransformerException: java.lang.NullPointerException
	at com.sun.org.apache.xalan.internal.xsltc.trax.TransformerImpl.transform(TransformerImpl.java:754)
	at com.sun.org.apache.xalan.internal.xsltc.trax.TransformerImpl.transform(TransformerImpl.java:359)
	at TestTrans.main(TestTrans.java:16)
Caused by: java.lang.NullPointerException
	at com.sun.org.apache.xalan.internal.lib.ExsltSets.leading(ExsltSets.java:63)
	at librarySet01.template$dot$0()
	at librarySet01.applyTemplates()
	at librarySet01.transform()
	at com.sun.org.apache.xalan.internal.xsltc.runtime.AbstractTranslet.transform(AbstractTranslet.java:619)
	at com.sun.org.apache.xalan.internal.xsltc.trax.TransformerImpl.transform(TransformerImpl.java:747)
	... 2 more
---------
java.lang.NullPointerException
	at com.sun.org.apache.xalan.internal.lib.ExsltSets.leading(ExsltSets.java:63)
	at librarySet01.template$dot$0()
	at librarySet01.applyTemplates()
	at librarySet01.transform()
	at com.sun.org.apache.xalan.internal.xsltc.runtime.AbstractTranslet.transform(AbstractTranslet.java:619)
	at com.sun.org.apache.xalan.internal.xsltc.trax.TransformerImpl.transform(TransformerImpl.java:747)
	at com.sun.org.apache.xalan.internal.xsltc.trax.TransformerImpl.transform(TransformerImpl.java:359)
	at TestTrans.main(TestTrans.java:16)


In XSL, select /doc/non-existing shoud returns an empty nodelist instance, but returns null actually, that causes NPE.