Fixed the null user problem again

Basically the null user problem that caused #3 and #4 was persisting, because in `Ravenly::user()` the null check didn't check the attributes of the user. Created `Ravenly::isValidUser($user)` to do the null checking and put it all in place.

The problem shouldn't persist anymore.

Signed-off-by: Gideon Farrell <me@gideonfarrell.co.uk>