dnsrecord: Treat empty list arguments correctly
    
    dnsrecord_del fails when one of the record arguments is an empty list:
    
        AttrValueNotFound("AAAA record does not contain 'None'",)
    
    The problem is caused by the fact that LDAPEntry.__getitem__ returns None
    for empty lists. The code in the plugin considers None as a single entry
    and maps it to vals = [None].
    
    The patch maps None to empty list.
    
    Fixes: https://pagure.io/freeipa/issue/8196
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>