We disallow empty strings for `href` and `src` since they're common
mistakes that end up loading the current page as a preload, image or
link. We also disallow it for `action`. You have to pass `null` which is
the same.
However, for `formAction` passing `null` is not the same as passing
empty string. Passing empty string overrides the form's action to be the
current page even if the form's action was set to something else.
There's no easy way to express the same thing `#` show up in the user
visible URLs and `?` clears the search params.
Since this is also not a common mistake, we can just allow this.
Sebastian Markbåge
2788d0d8dd