Don't try to select a nav item if the selected item is explicitly null

Summary: If there are items (like labels) without keys, we incorrectly select the first item with no key in response to a `selectFilter(null)` call.

Test Plan: Called `selectFilter(null)`, didn't get a selected label.

Reviewers: chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D4428