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

lib/active_model_serializers/adapter/json_api/relationship.rb

@@ -24,6 +24,7 @@ module ActiveModelSerializers
 
           meta = meta_for(association)
           hash[:meta] = meta if meta
+          hash[:meta] = {} if hash.empty?
 
           hash
         end