In select helper, '_default argument should be disabled

[adrianhurt]

As de docs says, the '_default argument should always appear as disabled. But the doc's example is not disabled at all. Check if it's a bug on the docs or on the code.

Copied from docs:

You can add a default first value using the argument '_default with a string. It will add a first option to the select with an empty string value. So you could add a required constraint to the field to force the user to select one other option. In addition, this default option is always disable to avoid the user to select it, and if any other option is selected this default one will not appear at all.