ChangeBundle: Ignore null originalSubject in ReviewDb changes

Original subject is a nullable field added relatively recently, so
there are many changes that don't have it set. In NoteDb we
reconstruct this by looking at the patch set commit, so it is always
present. Don't consider this a difference between ReviewDb and NoteDb.

Change-Id: I143430fed2da531806f2b8dcc17465f827df80d7

gerrit-reviewdb/src/main/java/com/google/gerrit/reviewdb/client/Change.java

@@ -572,6 +572,10 @@ public final class Change {
     return originalSubject != null ? originalSubject : subject;
   }
 
+  public String getOriginalSubjectOrNull() {
+    return originalSubject;
+  }
+
   /** Get the id of the most current {@link PatchSet} in this change. */
   public PatchSet.Id currentPatchSetId() {
     if (currentPatchSetId > 0) {