Fixed a bug that appears when a nil association is included

[groyoh]

Take a serializer that has an association with a serializer specified for this association, e.g.:

PostPreviewSerializer = Class.new(ActiveModel::Serializer) do
  attributes :title, :body, :id
  belongs_to :author, serializer: AuthorPreviewSerializer
end

When serialized with JsonApi adapter, if this association is included and the value associated is nil then included would look like this: "included":[{"id":"", type:"nil_classes"}].

I changed the behaviour so that it only non nil values are added to included.

[kurko]

Thanks @groyoh!