Currently when no report files are found we get the following console output and a failed build:
FATAL: No NUnit test report files were found. Configuration error?
However because our build job is parameterised, we do not always generate test results, and there is no way to parameterise the "Public NUnit test result report" Post-build Action.
It would make sense for there to be an advanced option that does not fail the build if no reports were found, much like for the "Archive the artifacts" Post-build Action.