Fix favicon 404s triggered when using loginAs() or logout() methods

[ziadoz]

Every Dusk test that uses loginAs() or logout() appears to record the following error in tests/Browser/console:

[
    {
        "level": "SEVERE",
        "message": "https:\/\/localhost\/favicon.ico - Failed to load resource: the server responded with a status of 404 ()",
        "source": "network",
        "timestamp": 1695715357814
    }
]

The referrer for the 404 is https://localhost/_dusk/login/{id}.

It happens because the controller than handles logging in and out returns void, which is presumably treated as an empty 200 response. This causes Chrome to make an additional request to favicon.ico. Instead these methods should return a 204 no content response, which informs Chrome it doesn't need to and stops these error logs.

This bug caused me a bit of confusion, as for a while I thought my app was missing a favicon somehow when it wasn't.