serializable_hash and as_json should take options = nil

per ActiveModel::Serialization#serializable_hash
96bb004fc6/activemodel/lib/active_model/serialization.rb
    def serializable_hash(options = nil)
          options ||= {}

Otherwise, passing in nil to `as_json` or `serializable_hash`
makes things blow up when passing nil into attributes

lib/active_model/serializer/adapter/null.rb

@@ -2,7 +2,7 @@ module ActiveModel
   class Serializer
     class Adapter
       class Null < Adapter
-        def serializable_hash(options = {})
+        def serializable_hash(options = nil)
           {}
         end
       end