Hide URL button when the field is empty

The disabled attribute does not prevent the user to click on the empty link.

Closes #12747

sao/src/view/form.js

@@ -4588,7 +4588,7 @@ function eval_pyson(value){
         },
         set_url: function(value) {
             this.button.attr('href', value);
-            this.button.prop('disabled', !value);
+            this.button.toggle(value);
         },
         set_invisible: function(invisible) {
             Sao.View.Form.URL._super.set_invisible.call(this, invisible);