Fix PHP 8.1 "strlen(null)" exception on "Link External Account" page at login

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.

```
EXCEPTION: (RuntimeException) strlen(): Passing null to parameter freebsd#1 ($string) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(), phorge()
  #0 <freebsd#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/applications/auth/controller/PhabricatorAuthSetExternalController.php:43]
```

Closes T15606

Test Plan: Follow steps in T15606; page `/auth/external/` displays as expected the content: "Link External Account - You can link your Phabricator account to an external account to allow you to log in more easily in the future. To continue, choose an account to link below. If you prefer not to link your account, you can skip this step." and the "Link External Account" button.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: avivey, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15606

Differential Revision: https://we.phorge.it/D25395

Author: aklapper

Diff for: src/applications/auth/controller/PhabricatorAuthSetExternalController.php

@@ -40,7 +40,7 @@ public function handleRequest(AphrontRequest $request) {
     $text = PhabricatorAuthMessage::loadMessageText(
       $viewer,
       PhabricatorAuthLinkMessageType::MESSAGEKEY);
-    if (!strlen($text)) {
+    if (!phutil_nonempty_string($text)) {
       $text = pht(
         'You can link your %s account to an external account to '.
         'allow you to log in more easily in the future. To continue, choose '.