s3:winbind_group: fix "getent group" to allocate new gids.

s3:winbind_group: fix "getent group" to allocate new gids.

"getent group" used to fill the idmap cache with negative
cache entries for unmapped group sids.

Don't pass domain name unconditionally to idmap_sid_to_gid().
idmap_sid_to_gid() only creates new mappings (allocating
idmap backends tdb, tdb2, ldap...) when the domain name passed
in is "".

Note that it is _wrong_ to directly call the idmap_sid_to_gid()
functions here, in the main winbindd. The correct fix would be
to send a sid_to_gid request to winbindd itself, but this needs
more work to prepare the async mechanisms, and we nee a quick
fix for getent passwd now.

Michael