Fix PHP 8.1 "strlen(null)" exception adding LDAP authentication

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 #1 ($string) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(head=master, ref.master=788098096e11), phorge(head=master, ref.master=840a7fab2bc8)
  #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/applications/auth/provider/PhabricatorLDAPAuthProvider.php:430]
```

Closes T15587

Test Plan: Go to `/auth/config/edit/?provider=PhabricatorLDAPAuthProvider` and see that "Add Auth Provider: LDAP" page is rendered.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15587

Differential Revision: https://we.phorge.it/D25372

src/applications/auth/provider/PhabricatorLDAPAuthProvider.php

@@ -427,7 +427,7 @@ final class PhabricatorLDAPAuthProvider extends PhabricatorAuthProvider {
       }
 
       $instruction_text = idx($instructions, $key);
-      if (strlen($instruction_text)) {
+      if (phutil_nonempty_string($instruction_text)) {
         $form->appendRemarkupInstructions($instruction_text);
       }