R1:ee884db1f935

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…