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.