[jenkins] Don't try to upload any packaged Xamarin.Mac tests if there aren't any or they failed to compile. (#8678)

jenkins/Jenkinsfile

@@ -834,9 +834,12 @@ timestamps {
                                     hasXamarinMacTests = false
                                     echoError ("Failed to package Xamarin.Mac tests (exit code: ${exitCode})")
                                     failedStages.add (currentStage)
-                                }
+                                } else {
+                                    def packaged_xm_tests = findFiles (glob: "tests/*.7z")
+                                    if (packaged_xm_tests.size () > 0)
                                         uploadFiles ("tests/*.7z", "wrench", virtualPath)
                                 }
+                            }
 
                             timeout (time: 13, unit: 'HOURS') {
                                 // We run tests locally and on older macOS bots in parallel.