sd-netlink: don't treat type_system->count==0 as invalid

Empty type-systems are just fine. Avoid the nasty hack in
union-type-systems that treat empty type-systems as invalid. Instead check
for the actual types-array and make sure it's non-NULL (which is even true
for empty type-systems, due to "empty_types" array).

src/libsystemd/sd-netlink/netlink-types.c

@@ -598,7 +598,7 @@ int type_system_union_protocol_get_type_system(const NLTypeSystemUnion *type_sys
                 return -EOPNOTSUPP;
 
         type_system = &type_system_union->type_systems[protocol];
-        if (type_system->count == 0)
+        if (!type_system->types)
                 return -EOPNOTSUPP;
 
         *ret = type_system;