Edit story: can't deselect a story author to return to 'Select an author'

## Problem
On an already-created unpublished story, there's no way to deselect the story author and return to the "Select an author" placeholder. The deselect behavior works on a new story form but not when editing an existing one.

## Expected behavior
The author field should be clearable on the edit form the same way it is on the new story form.