Ensure valid jsonapi when blank relationship (#1930)

If you specify include_data false, and do not have any links for this
relationship, we would output something like:

`{ relationships: { comments: {} } }`

This is not valid jsonapi. We will now render

`{ relationships: { comments: { meta: {} } } }`

Instead.

Relevant jsonapi spec: http://jsonapi.org/format/#document-resource-object-relationships

test/adapter/json_api/relationship_test.rb

@@ -54,7 +54,7 @@ module ActiveModelSerializers
         end
 
         def test_relationship_data_not_included
-          test_relationship({}, options: { include_data: false })
+          test_relationship({ meta: {} }, options: { include_data: false })
         end
 
         def test_relationship_simple_link