git-cl-upload: Fix reviewers notification with --no-squash.

Using --no-squash resulted in an exception being thrown, because issue
number is not set in that case.

Change-Id: Iaf0d7eb05851eba0d5231640c077b845bca486ee
Reviewed-on: https://chromium-review.googlesource.com/505509
Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Wiktor Garbacz <wiktorg@google.com>

gerrit_util.py

@@ -660,6 +660,8 @@ def AddReviewers(host, change, reviewers=None, ccs=None, notify=True,
   """Add reviewers to a change."""
   if not reviewers and not ccs:
     return None
+  if not change:
+    return None
   reviewers = frozenset(reviewers or [])
   ccs = frozenset(ccs or [])
   path = 'changes/%s/revisions/current/review' % change