[rbslack] Fix a crash when review requests are closed/reopend by hooks.

Review Request #6296 — Created Sept. 4, 2014 and submitted — Latest diff uploaded




When closing a review request through a post-commit hook, or any other
means where a user isn't provided, rbslack would crash. Same with
reopening without a provided user. Now, if the user is None, it will
default to the owner of the review request.

The other signals all have a valid user passed in, so they don't need
this sort of check.

Forced the user to None when calling the signals for closing and reopening.
Saw it default to the user.