In case of No Tests were run, or All Tests were Skipped, TestNG Plugin recognizes it as:
"Found matching files but did not find any TestNG results."
and treats the build as Success - whereas it should be marked as failure.

Here's an example:

...
TestNG Reports Processing: START
Looking for TestNG results report in workspace using pattern: **/testng-results.xml
Saving reports...
Processing 'C:\Jenkins\jobs\Automation_Matrix_New\configurations\axis-BROWSER\FIREFOX\axis-SERVER\AUTOJEN1\axis-XML_Test_File\test_suites_QA_Trails.xml\builds\523\testng\testng-results.xml'
Found matching files but did not find any TestNG results.
Started calculate disk usage of build
Finished Calculation of disk usage of build in 0 seconds
Started calculate disk usage of workspace
Finished Calculation of disk usage of workspace in 0 seconds
Finished: SUCCESS