When evaluating NUnit3 reports, warnings appear in the Jenkins log.

We use the following evaluation:

NUnit3(pattern: 'Logs/UnitTestResults/*_nunit.xml'),
 
This generates the warning:

Warning at function xunit:is-empty in line 51
  SXWN9000 A function that calculates atomic values should use xsl:sequence and not xsl:value-of

It appears to be the xunit:is-empty function in XSLT.

https://github.com/jenkinsci/xunit-plugin/blob/master/src/main/resources/org/jenkinsci/plugins/xunit/types/nunit-3.0-to-junit.xsl

    <xsl:function name="xunit:is-empty" as="xs:boolean">
        <xsl:param name="value" as="xs:string?" />
        <xsl:value-of select="string($value) != ''" />
    </xsl:function>