Fixed wrong variable name on assertNotNullOrEmpty_
Fixed wrong variable name on assertNotNullOrEmpty_

While testing the workflow, the ```gulp upload-latest --env=test``` step displayed a warning:

    src\tests\lib.testing.js
      line 68  col 24  'message' used out of scope.
    
      ‼  1 warning
gsuitedevs/apps-script-samples,gsuitedevs/apps-script-samples,googleworkspace/apps-script-samples,roofcat/google-apps-script-samples,googleworkspace/apps-script-samples