idrange-add: properly handle empty --dom-name option
    
    When idrange-add is called with --dom-name=, the CLI exits with
    ipa: ERROR: an internal error has occurred
    This happens because the code checks if the option is provided but does not
    check if the value is None.
    
    We need to handle empty dom-name as if the option was not specified.
    
    https://pagure.io/freeipa/issue/6404
    
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>