Fix constant lookup when serializer is not available.

test/serializer_support_test.rb

@@ -0,0 +1,11 @@
+require "test_helper"
+
+class RandomModel
+  include ActiveModel::SerializerSupport
+end
+
+class SerializerSupportTest < ActiveModel::TestCase
+  test "it returns nil if no serializer exists" do
+    assert_equal nil, RandomModel.new.active_model_serializer
+  end
+end