[METASVC-149] Add condition to subjects_is_unique_type_identifier

If there are no identifiers then type and label must be unique
among those entries without identifiers. Therefore entries are allowed
that have the same type and label but one is with and one w/o
identifiers.

metasvc-server/backend-jdbi/src/main/resources/io/github/dbmdz/metadata/server/backend/impl/database/migration/R__func_subjects_is_unique_type_identifier.sql

@@ -14,7 +14,7 @@ declare
 	tuple_exists boolean;
 begin
 	if ids is null or cardinality(ids) = 0 then
-		execute 'select exists(select 1 from public.subjects where $1 = type and $2 = label and $3 <> uuid)'
+		execute 'select exists(select 1 from public.subjects where $1 = type and $2 = label and coalesce(identifiers, array[]::dbidentifier[]) = array[]::dbidentifier[] and $3 <> uuid)'
 		  into tuple_exists
 		  using ptype, plabel, puuid;
 		if tuple_exists then