return nil if id is nil

lib/active_model_serializers/adapter/json_api/relationship.rb

@@ -55,7 +55,6 @@ module ActiveModelSerializers
               else
                 association.reflection.type.to_s
               end
-            # TODO: probably return nil if association.object is nil?
             ResourceIdentifier.for_type_with_id(type, id, serializable_resource_options)
           else
             # TODO(BF): Process relationship without evaluating lazy_association

lib/active_model_serializers/adapter/json_api/resource_identifier.rb

@@ -13,7 +13,7 @@ module ActiveModelSerializers
           type = inflect_type(type)
           type = type_for(:no_class_needed, type, options)
           if id.blank?
-            { type: type }
+            nil
           else
             { id: id.to_s, type: type }
           end

test/adapter/json_api/relationship_test.rb

@@ -36,6 +36,7 @@ module ActiveModelSerializers
         end
 
         def test_relationship_with_nil_model_and_belongs_to_id_on_self
+          original_config = ActiveModelSerializers.config.jsonapi_use_foreign_key_on_belongs_to_relationship
           ActiveModelSerializers.config.jsonapi_use_foreign_key_on_belongs_to_relationship = true
 
           expected = { data: nil }
@@ -46,7 +47,10 @@ module ActiveModelSerializers
           actual = build_serializer_and_serialize_relationship(model, relationship_name) do
             belongs_to :blog
           end
+
           assert_equal(expected, actual)
+        ensure
+          ActiveModelSerializers.config.jsonapi_use_foreign_key_on_belongs_to_relationship = original_config
         end
 
         def test_relationship_with_data_array

test/adapter/json_api/type_test.rb

@@ -147,8 +147,7 @@ module ActiveModelSerializers
         def test_for_type_with_id_given_blank_id
           id = ''
           actual = ResourceIdentifier.for_type_with_id('admin_user', id, {})
-          expected = { type: 'admin-users' }
-          assert_equal actual, expected
+          assert_nil actual
         end
 
         def test_for_type_with_id_inflected