Fix PHP 8.1 "strlen(null)" exception which blocks rendering user's Edit Profile page

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.

Closes T15319

Test Plan:
Applied this change (on top of D25147, D25153, D25167) and
`/people/editprofile/userid/` finally rendered in web browser.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15319

Differential Revision: https://we.phorge.it/D25168

src/infrastructure/customfield/field/PhabricatorCustomFieldList.php

@@ -89,7 +89,7 @@ final class PhabricatorCustomFieldList extends Phobject {
       $field_handles = array_select_keys($handles, $phids[$field_key]);
 
       $instructions = $field->getInstructionsForEdit();
-      if (strlen($instructions)) {
+      if (phutil_nonempty_string($instructions)) {
         $form->appendRemarkupInstructions($instructions);
       }