Don't include empty polymoprhic associations

Take this serializer:

class TodoSerializer < ActiveModel::Serializer
  root :todo, :include => true
  has_one :reference, :polymorphic => true
end

A nil reference would generate this JSON:

{
  "todo": { "reference": null },
  "nil_classes": []
}

This commit prevents the `nil_classes` key from being added when
serializing and including nil polymoprhic associations.

lib/active_model/serializer.rb

@@ -194,6 +194,10 @@ module ActiveModel
           option(:include, source_serializer._root_embed)
         end
 
+        def embeddable?
+          !associated_object.nil?
+        end
+
       protected
 
         def find_serializable(object)
@@ -228,6 +232,14 @@ module ActiveModel
       end
 
       class HasOne < Config #:nodoc:
+        def embeddable?
+          if polymorphic? && associated_object.nil?
+            false
+          else
+            true
+          end
+        end
+
         def polymorphic?
           option :polymorphic
         end
@@ -516,7 +528,7 @@ module ActiveModel
 
         if association.embed_in_root? && hash.nil?
           raise IncludeError.new(self.class, association.name)
-        elsif association.embed_in_root?
+        elsif association.embed_in_root? && association.embeddable?
           merge_association hash, association.root, association.serialize_many, unique_values
         end
       elsif association.embed_objects?