Handle the no message case in _deduplicate_testresults.
Handle the no message case in _deduplicate_testresults.

Summary: The message field can be None which is causing some errors (saw them on Sentry).

Reviewers: kylec

Reviewed By: kylec

Subscribers: changesbot

Differential Revision: https://tails.corp.dropbox.com/D165389

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