[3.0] Answer an activation link that names nobody

[albertlast]

Description

An activation link whose u names nobody is a 500.

$ curl -s -o /dev/null -w '%{http_code}\n' 'index.php?action=activate;u=999;code=x'
500
$ mysql -e "SELECT message, file, line FROM smf_log_errors ORDER BY id_error DESC LIMIT 1"
Typed property SMF\Actions\Activate::$member must not be accessed before initialization
/var/www/html/Sources/Actions/Activate.php   255

Activate::loadMember() assigns $this->member only when it finds someone. The
line after the call reads $this->member->is_activated unconditionally.

The handling for this already exists and cannot be reached.
Activate::execute() opens with

if (!isset($this->member)) {
    if (empty($_REQUEST['u']) && empty($_POST['user'])) {
        $this->showResendRequest();
    } else {
        $this->showRetryInvalidUser();
    }

    return;
}

which is 2.1's behaviour — the form asking for a username and an activation code.
The constructor throws first, so execute() never runs.

This is reachable in ordinary use: a member deleted while their activation mail
is in flight, an id that got mangled in a mail client, or a link followed after
the account was pruned.

What changes

The constructor returns when loadMember() found nothing, and leaves the answer
to the guard execute() already has.

Testing

?action=activate;u=999;code=x and ?action=activate;u=999 both give a 200 with
"User does not exist" and the retry form, and log nothing. A real activation link
still activates; ?action=activate with no u still shows the resend form.

Issues References (Fixes|Related|Closes)

Related to #7933