Handle response_body= when body is nil

There are some cases when the `body` in `response_body=` can be set to
nil. One of those cases is in `actionpack-action_caching` which I found
while upgrading it for Rails 5.

It's not possible to run `body.each` on a `nil` body so we have to
return after we run `response.reset_body!`.

actionpack/lib/action_controller/metal.rb

@@ -174,6 +174,7 @@ module ActionController
     def response_body=(body)
       body = [body] unless body.nil? || body.respond_to?(:each)
       response.reset_body!
+      return unless body
       body.each { |part|
         next if part.empty?
         response.write part

actionpack/test/controller/new_base/bare_metal_test.rb

@@ -40,6 +40,22 @@ module BareMetalTest
     end
   end
 
+  class BareEmptyController < ActionController::Metal
+    def index
+      self.response_body = nil
+    end
+  end
+
+  class BareEmptyTest < ActiveSupport::TestCase
+    test "response body is nil" do
+      controller = BareEmptyController.new
+      controller.set_request!(ActionDispatch::Request.empty)
+      controller.set_response!(BareController.make_response!(controller.request))
+      controller.index
+      assert_equal nil, controller.response_body
+    end
+  end
+
   class HeadController < ActionController::Metal
     include ActionController::Head