Fix exception for Revert Submission

Enclosed changes that had no submission id used to
throw Null pointer exception. This change throws a
meaningful message instead.

Change-Id: I1b3f807d682310216ea3157ebfcbe5e98cdb7a1b

java/com/google/gerrit/server/restapi/change/RevertSubmission.java

@@ -95,11 +95,12 @@ public class RevertSubmission
           String.format("change is %s.", ChangeUtil.status(changeResource.getChange())));
     }
 
-    String submissionId =
-        requireNonNull(
-            changeResource.getChange().getSubmissionId(),
-            String.format("merged change %s has no submission ID", changeResource.getId()));
-
+    String submissionId = changeResource.getChange().getSubmissionId();
+    if (submissionId == null) {
+      throw new ResourceConflictException(
+          "This change is merged but doesn't have a submission id,"
+              + " meaning it was not submitted through Gerrit.");
+    }
     List<ChangeData> changeDatas = queryProvider.get().bySubmissionId(submissionId);
 
     for (ChangeData changeData : changeDatas) {