Ticket 48665 - Prevent sefault in ldbm_instance_modify_config_entry
    
    Bug Description:  python ldap sends a mod_delete then a mod_add to the server.
    This causes a segfault in ldbm_instance_modify_config_entry which expects the
    value of the delete operation to exist.
    
    Fix Description:  If there is no attribute in the delete operation, this now
    passes a NULL to ldbm_config_set, which then rejects the op as NO_SUCH_ATTRIBUTE
    
    There is an attached test case, which also checks that mod_replace is still
    functional.
    
    https://fedorahosted.org/389/ticket/48665
    
    Author: wibrown
    
    Review by: nhosoi (Thanks!)