Fix a couple small issues in the new related user selector.
Review Request #8248 — Created June 17, 2016 and submitted
The user selector worked pretty well for most cases, but I encountered a couple
small issues when saving certain forms with empty lists of users. This change
fixes them.
Saved a bunch of forms. No longer see weird errors about the empty string not
being a valid pk.
Tool: PEP8 Style Checker Processed Files: reviewboard/admin/form_widgets.py Tool: Pyflakes Processed Files: reviewboard/admin/form_widgets.py