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

Review Request #8248 — Created June 17, 2016 and submitted — Latest diff uploaded

Information

Review Board
release-2.5.x
8703e77...

Reviewers

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.