fix error where blank email addresses were sneaking in the stack

Summary: array_filter to the rescue

Test Plan: mostly lots of reasoning (as opposed to making a fake user with a blank email address to reproduce)

Reviewers: epriestley, chad

Reviewed By: chad

CC: aran, Korvin

Maniphest Tasks: T2052

Differential Revision: https://secure.phabricator.com/D3927