Fix dropdown for file whose diff is not displayable

Summary: The current behavior is that, when the dropdown menu is clicked for a binary file, it navigates to the current revision page directly without showing the dropdown meu. The fix is to do nothing when there is no file displayed.

Test Plan:

• checked a diff with binary file
• checked a diff with normal files

Reviewers: vrana, epriestley

Reviewed By: epriestley

CC: epriestley, aran

Differential Revision: https://secure.phabricator.com/D2801