Skip SubmitRecords that are missing labels

A record cna be missing labels if the change is closed, or the patch
set is not the current patch set, etc. Skip these rather than NPE.

Change-Id: Iaa5d32ee9a7279a66535c66d3ef6eb05efd3a837

gerrit-server/src/main/java/com/google/gerrit/server/change/ReviewerJson.java

@@ -109,6 +109,9 @@ public class ReviewerJson {
     PatchSet ps = cd.currentPatchSet(db);
     if (ps != null) {
       for (SubmitRecord rec : ctl.canSubmit(db.get(), ps, cd, true, false)) {
+        if (rec.labels == null) {
+          continue;
+        }
         for (SubmitRecord.Label label : rec.labels) {
           String name = label.label;
           if (!out.approvals.containsKey(name)