fix incorrect suffix handling in topology checks
    
    When trying to delete a partially removed master entry lacking
    'iparepltopomanagedsuffix' attribute, the code that tries to retrieve
    tha value for further computations passes None and causes unhandled
    internal errors.
    
    If the attribute is empty or not present, we should return empty list
    instead as to not break calling cod attribute, the code that tries to
    retrieve tha value for further computations passes None and causes
    unhandled internal errors. We should return empty list instead.
    
    https://pagure.io/freeipa/issue/6965
    
    Reviewed-By: Felipe Volpone <felipevolpone@gmail.com>