replmd: treat a zero GUID as not present in get_parsed_dns

replmd: treat a zero GUID as not present in get_parsed_dns

This roughly follows the pattern in the 2009 commit
0d5d7f58473c989bff4 by the Andrews Tridgell and Bartlett, which dealt
with zero GUIDs in replmd_add_fix_la(). That function is about to use
get_parsed_dns() [see next commit], and the other users of
get_parsed_dns don't really want to see zero guids, so it is simpler
to test here.

This makes hitting the GUID_all_zero branch of parsed_dn_find() even
more unlikely.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Pair-programmed-with: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>