Bug 1332844: allow printing Options when notifications is None; r=gbrown

(I stumbled over this while debugging)

MozReview-Commit-ID: GCIMv3Zhbp8

--HG--
extra : rebase_source : f80dbdc252726e28c72b5bb381441ba2dffb70a4

taskcluster/taskgraph/try_option_syntax.py

@@ -559,5 +559,5 @@ class TryOptionSyntax(object):
             "jobs: " + none_for_all(self.jobs),
             "trigger_tests: " + str(self.trigger_tests),
             "interactive: " + str(self.interactive),
-            "notifications: " + self.notifications,
+            "notifications: " + str(self.notifications),
         ])