Fix PHP 8.1 "strlen(null)" exception in PhabricatorAuthAccountView

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 freebsd#1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/auth/view/PhabricatorAuthAccountView.php:66]
```

Closes T15900

Test Plan: Try to register an account via LDAP.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15900

Differential Revision: https://we.phorge.it/D25761

Author: aklapper

src/applications/auth/view/PhabricatorAuthAccountView.php

@@ -63,7 +63,7 @@ public function render() {
       ));
 
     $account_uri = $account->getAccountURI();
-    if (strlen($account_uri)) {
+    if (phutil_nonempty_string($account_uri)) {
 
       // Make sure we don't link a "javascript:" URI if a user somehow
       // managed to get one here.