Fix a couple small issues in the new related user selector.
Fix a couple small issues in the new related user selector.

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.

Testing done:
Saved a bunch of forms. No longer see weird errors about the empty string not
being a valid pk.

Reviewed at https://reviews.reviewboard.org/r/8248/

reviewboard/reviewboard,brennie/reviewboard,reviewboard/reviewboard,brennie/reviewboard,chipx86/reviewboard,sgallagher/reviewboard,chipx86/reviewboard,chipx86/reviewboard,davidt/reviewboard,davidt/reviewboard,sgallagher/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,sgallagher/reviewboard,brennie/reviewboard,brennie/reviewboard,sgallagher/reviewboard,davidt/reviewboard,davidt/reviewboard