Simplify notify behavior in git-cl

Change-Id: I872658970c04cdadda2b0eaffd95e10717c09493
Reviewed-on: https://chromium-review.googlesource.com/575009
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>
diff --git a/gerrit_util.py b/gerrit_util.py
index 1756a13..7afe96b 100755
--- a/gerrit_util.py
+++ b/gerrit_util.py
@@ -717,7 +717,7 @@
     body['message'] = msg
   if labels:
     body['labels'] = labels
-  if notify:
+  if notify is not None:
     body['notify'] = 'ALL' if notify else 'NONE'
   if ready:
     body['ready'] = True