Running xunit with NUnit-Report shows a warning.

Warning at function xunit:is-empty on line 51
  SXWN9000  A function that computes atomic values should use xsl:sequence rather than xsl:value-of

It seems this is the problematic line in nunit-3.0-to-junit.xsl:

https://github.com/jenkinsci/xunit-plugin/blob/435401a04c6c4a988ec393506c71ddc1af8715b0/src/main/resources/org/jenkinsci/plugins/xunit/types/nunit-3.0-to-junit.xsl#L53

Change the result of function xunit:is-empty to xsl:sequence instead of using xsl-value-of.