Do not crash on empty reverse member options
    
    Calling a LDAP{Add,Remove}ReverseMember with an empty reverse_member
    caused an internal error, because empty values are converted to None,
    which is then iterated.
    
    Use an empty list instead of None (or other false falues, of which we
    only use the empty list).
    
    https://fedorahosted.org/freeipa/ticket/2681