Return None if the project doesn't exist when looking for contributors
    
    When looking for contributors in pagure project let's return None if the request
    returns 404, because this means that the project doesn't exist.
    
    This will allow scm_request_processor to stop the processing of the request for
    new branch at start of the processing.
    
    Signed-off-by: Michal Konečný <mkonecny@redhat.com>