Only set a user's password during unit tests if one is provided.
Only set a user's password during unit tests if one is provided.

`TestCase.create_user()` was unconditionally setting a user's password,
but if a password wasn't provided, the end result wasn't really correct.
We ran into this on RBCommons and fixed it there, but this fix is worth
backporting to Review Board.

This change simply makes the `set_password()` call conditional on
whether a password was provided.

Testing Done:
Unit tests passed.

Reviewed at https://reviews.reviewboard.org/r/10887/

chipx86/reviewboard,chipx86/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard