Fix PHP 8.1 "strlen(null)" exception which blocks creating a project with an empty Description field

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 second half of T15331

Test Plan: Applied these two changes (one in Arcanist, one in Phorge). Project with empty Description field was created and `/project/view/projectid/` 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: T15331

Differential Revision: https://we.phorge.it/D25177

src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldRemarkup.php

@@ -30,7 +30,7 @@ final class PhabricatorStandardCustomFieldRemarkup
   public function renderPropertyViewValue(array $handles) {
     $value = $this->getFieldValue();
 
-    if (!strlen($value)) {
+    if (!phutil_nonempty_string($value)) {
       return null;
     }