After creating an article and the URL field is left empty, when editting the node, you cant add a new URL

I have the "URL field is in the form, but it can be left empty" option on, so my users can add external articles and also create their own original articles.

The problem is that when a user publishes an article and "forgets" to enter a URL, when he tries to edit the article to correct it and add the URL, the URL field is gone.

It would be nice if the url field is still there when you edit an article independently if you added a URL or not when you first published the article.

Thanks in advance!!! :)