test presence (catch empty strings too) of linkage_id and linkage_type

Author: barelyknown

lib/jsonapi/resource_serializer.rb

@@ -353,8 +353,10 @@ def related_link(source, relationship)
     end
 
     def to_one_linkage(source, relationship)
-      return unless linkage_id = foreign_key_value(source, relationship)
-      return unless linkage_type = format_key(relationship.type_for_source(source))
+      linkage_id = foreign_key_value(source, relationship)
+      linkage_type = format_key(relationship.type_for_source(source))
+      return unless linkage_id.present? && linkage_type.present?
+
       {
         type: linkage_type,
         id: linkage_id,