Fix PHP 8.1 "strlen(null)" exception in PhabricatorAuthStartController.php

Summary:
`strlen()` was used in Phabricator to check if a generic value is a non-empty string.
This behavior is deprecated since PHP 8.1. Phorge adopts `phutil_nonempty_string()` as a replacement.

Note: this may highlight other absurd input values that might be worth correcting
instead of just ignoring. If phutil_nonempty_string() throws an exception in your
instance, report it to Phorge to evaluate and fix that specific corner case.

```
ERROR 8192: strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/auth/controller/PhabricatorAuthStartController.php:34]
```

Closes T15832

Test Plan: Run `arc unit` locally, probably also don't be logged in.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15832

Differential Revision: https://we.phorge.it/D25639

src/applications/auth/controller/PhabricatorAuthStartController.php

@@ -31,7 +31,7 @@ final class PhabricatorAuthStartController
     $session_token = $request->getCookie(PhabricatorCookies::COOKIE_SESSION);
     $did_clear = $request->getStr('cleared');
 
-    if (strlen($session_token)) {
+    if (phutil_nonempty_string($session_token)) {
       $kind = PhabricatorAuthSessionEngine::getSessionKindFromToken(
         $session_token);
       switch ($kind) {