R1:deed4ffed0f6

Don't output empty summary in e-mails

Summary:
Reviews with empty summary are rendered like this:

Reviewers: ...

TEST PLAN

Test Plan:
Use empty summary.
Use non-empty summary.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1528