Change Request#media_type to return nil

When the request don't have a Content-Type header we were returning
an empty string instead of nil like Rack does.

actionpack/CHANGELOG.md

@@ -1,4 +1,6 @@
-## Unreleased
+*   Change `ActionDispatch::Request#media_type` to return `nil` when the request don't have a `Content-Type` header.
+
+    *Rafael Mendonça França*
 
 *   Fix error in `ActionController::LogSubscriber` that would happen when throwing inside a controller action.
 

actionpack/lib/action_dispatch/http/request.rb

@@ -266,7 +266,7 @@ def original_url
     #    # get "/articles"
     #    request.media_type # => "application/x-www-form-urlencoded"
     def media_type
-      content_mime_type.to_s
+      content_mime_type&.to_s
     end
 
     # Returns the content length of the request as an integer.

actionpack/test/dispatch/request_test.rb

@@ -1301,7 +1301,7 @@ class RequestFormData < BaseRequestTest
   test "no Content-Type header is provided and the request_method is POST" do
     request = stub_request("REQUEST_METHOD" => "POST")
 
-    assert_equal "", request.media_type
+    assert_nil request.media_type
     assert_equal "POST", request.request_method
     assert_not_predicate request, :form_data?
   end