If match_entry == NULL all principals should be iterated.
Additionally this patch adds a check in ipadb_filter_escape() to make
sure that the input is not NULL.
Fixes: https://fedorahosted.org/freeipa/ticket/3011
Although the proper values for booleans from LDAP should be only uppercase,
389ds does allow wrong cased values without complaining. And we still have some
places where the wrong case is used.
Avoid getting frustrating errors when reading these values out.