flag_ci_pr: do not crash if the message does not have a commit_hash
    
    It appears that the commit_hash value sent by the CI system is actually
    coming from pagure's commit_stop which can be null/None.
    So, let's just check if the message has a commit_hash and if it does not
    we'll just bail to avoid keeping on crashing.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>