Optimise serializable_hash when options are empty

This reverts 8538dfdc08, which broke the
activemodel-serializers-xml gem.

We can still get most of the benefit by applying the optimisation from
7b39197742 to empty hashes as well as nil.
This has the additional benefit of retaining the optimisation when the
user passes an empty options hash.

activemodel/lib/active_model/serialization.rb

@@ -123,7 +123,7 @@ module ActiveModel
     def serializable_hash(options = nil)
       attribute_names = attributes.keys
 
-      return serializable_attributes(attribute_names) unless options
+      return serializable_attributes(attribute_names) if options.blank?
 
       if only = options[:only]
         attribute_names &= Array(only).map(&:to_s)
@@ -179,7 +179,7 @@ module ActiveModel
         return unless includes = options[:include]
 
         unless includes.is_a?(Hash)
-          includes = Hash[Array(includes).flat_map { |n| n.is_a?(Hash) ? n.to_a : [[n, nil]] }]
+          includes = Hash[Array(includes).flat_map { |n| n.is_a?(Hash) ? n.to_a : [[n, {}]] }]
         end
 
         includes.each do |association, opts|