Don't try and add null bug milestone to update milestones
    
    I'm not sure how, but on my local dev instance I got into a
    situation where we hit this block for a bug that didn't have a
    milestone, so we tried to put None into self.milestones here.
    That causes sqalchemy to blow up. So, let's protect against
    that just in case it somehow happens to the production instance
    later...
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>