Fixed a bug that appeared when json adapter serialize a nil association

Author: groyoh

lib/active_model/serializer/adapter/json.rb

@@ -23,7 +23,7 @@ def serializable_hash(options = {})
                   end
                 end
               else
-                if association
+                if association && association.object
                   @hash[name] = cache_check(association) do
                     association.attributes(options)
                   end

test/adapter/json/belongs_to_test.rb

@@ -34,6 +34,13 @@ def test_include_nil_author
 
             assert_equal({title: "Hello!!", body: "Hello, world!!", id: 43, comments: [], blog: {id: 999, name: "Custom blog"}, author: nil}, adapter.serializable_hash)
           end
+
+          def test_include_nil_author_with_specified_serializer
+            serializer = PostPreviewSerializer.new(@anonymous_post)
+            adapter = ActiveModel::Serializer::Adapter::Json.new(serializer)
+
+            assert_equal({title: "Hello!!", body: "Hello, world!!", id: 43, comments: [], author: nil}, adapter.serializable_hash)
+          end
         end
       end
     end