Summary:
The NUnit plugin may state that no test result file was found if a file exists, but contains no results.

Steps to reproduce

1. Create a project, pointing it to a repository which has no tests.
2. Configure the project to include executing Pester as a build step:

   powershell -noninteractive -Command "& 'Invoke-Command' -ScriptBlock {Import-Module Pester; Invoke-Pester -OutputFormat NUnitXml -OutputFile testresults.xml}" 
   

3. Configure the project to publish NUnit results from testresults.xml.

Expected Behavior:
An error is received stating that an XML file was found, but there is no test data in the XML file.

Actual Behavior:
The following error is received:
ERROR: Publisher 'Publish NUnit test result report' failed: No test report files were found. Configuration error?
This error indicates that there was no XML file generated, which can lead the user on a wild goose chase if there is an XML file with no test data in the workspace.