Minor, fix an issue with PhabricatorMenuView and default null keys.

This is kind of gross, but breaking some menus right now which end up with double-`''` keys. The current meaning of setKey(null) is different from not calling it (it means `setKey('')`).

This should be fixed more reasonably but there's a lot of legacy cruft in PhabricatorSideNavFilterView.

Auditors: btrahan