diffusion / differential - better handle case where no comment selected

Summary: Right now, if no comment is selected the JS executes and throws an exception. Instead, if nothing is selected just do nothing. Fixes T6107.

Test Plan: opened up a commit in diffusion with an inline comment. pressed 'r' and saw no exceptions and nothing happen. pressed 'n' to select the next inline comment and then 'r' and it worked. opened up a commit in diffusion without any inline comments. pressed 'r' and saw no exceptions and nothing happen. opened up a diff in differential with an inline comment. pressed 'r' and saw no exceptons and nothing happened. pressed 'n' to select the next inline comment and then 'r' and it worked.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6107

Differential Revision: https://secure.phabricator.com/D10843