Handle correctly null associations

null belongs_to associations are now serialized as nil instead
of raise an error during serialization.

test/adapter/json_api/belongs_to_test.rb

@@ -6,10 +6,15 @@ module ActiveModel
       class JsonApi
         class BelongsToTest < Minitest::Test
           def setup
+            @author = Author.new(id: 1, name: 'Steve K.')
             @post = Post.new(id: 42, title: 'New Post', body: 'Body')
+            @anonymous_post = Post.new(id: 43, title: 'Hello!!', body: 'Hello, world!!')
             @comment = Comment.new(id: 1, body: 'ZOMG A COMMENT')
             @post.comments = [@comment]
+            @anonymous_post.comments = []
             @comment.post = @post
+            @post.author = @author
+            @anonymous_post.author = nil
 
             @serializer = CommentSerializer.new(@comment)
             @adapter = ActiveModel::Serializer::Adapter::JsonApi.new(@serializer)
@@ -22,6 +27,13 @@ module ActiveModel
           def test_includes_linked_post
             assert_equal([{id: "42", title: 'New Post', body: 'Body'}], @adapter.serializable_hash[:linked][:posts])
           end
+
+          def test_include_nil_author
+            serializer = PostSerializer.new(@anonymous_post)
+            adapter = ActiveModel::Serializer::Adapter::JsonApi.new(serializer)
+
+            assert_equal({comments: [], author: nil}, adapter.serializable_hash[:posts][:links])
+          end
         end
       end
     end