How to reproduce from pipeline:

pipeline {
agent none
stages{
stage('Build') {
agent { label "Test" }
steps {
nunit failIfNoResults: true, testResultsPattern: '*.xml'
} // steps
} // stage
} // stages
} // pipeline

Drop attached TestResult.xml into %workspace% directory.

Current output: ERROR: Could not read the XSL XML file. Please report this issue to the plugin author

Expected output: File TestResult.xml has no test inside.