Don't fatal when viewing tags pointing at commits we haven't imported/parsed yet

Summary:
In Diffusion, the "Tags" view may read commits which haven't imported or parsed yet, and thus don't have loadable objects.

Most of this logic tests for if ($commit), but the author part did not. Instead, don't render author information if $commit is not present.

Test Plan:

• Loaded tags view with commits present.
• Faked $commit = null;, loaded tag view, got this instead of a fatal:

{F5068432}

Reviewers: chad, amckinley

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D18290