Add automated test of JENKINS-39044

If the ReverseTrigger threshold were null, a null pointer exception would
be thrown.  The config round trip operation resets a null threshold to
SUCCESS, so this test reaches inside the job definition and changes the
threshold directly.

Testing done:

* Confirmed that the modified automated test fails when the change to
  ReverseTrigger is removed.
* Confirmed that the modified automated test passes when the change to
  ReverseTrigger is included.

Author: MarkEWaite

test/src/test/java/jenkins/triggers/ReverseBuildTriggerTest.java

@@ -189,7 +189,7 @@ void upstreamProjectSecurity() throws Exception {
         assertEquals(3, downstream.getLastBuild().number);
     }
 
-    @Issue("JENKINS-29876")
+    @Issue({"JENKINS-29876", "JENKINS-39044"})
     @Test
     void nullJobInTriggerNotCausesNPE() throws Exception {
         final FreeStyleProject upstreamJob = r.createFreeStyleProject("upstream");
@@ -208,6 +208,11 @@ void nullJobInTriggerNotCausesNPE() throws Exception {
         r.configRoundtrip(downstreamJob2);
 
         r.jenkins.rebuildDependencyGraph();
+
+        // JENKINS-39044 - NPE if trigger threshold is null
+        ReverseBuildTrigger trigger = downstreamJob2.getTrigger(ReverseBuildTrigger.class);
+        trigger.setThreshold(null);
+
         final FreeStyleBuild build = r.buildAndAssertSuccess(upstreamJob);
         r.waitUntilNoActivity();