Fix PHP 8.1 "strlen(null)" exception in PhabricatorStandardCustomFieldLink.php

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 T15807

Test Plan: Unknown. Given that `$value` is later used to construct a `phutil_tag` I assume that it should be a string and not a scalar.

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

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

Maniphest Tasks: T15807

Differential Revision: https://we.phorge.it/D25610

src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldLink.php

@@ -21,7 +21,7 @@ final class PhabricatorStandardCustomFieldLink
   public function renderPropertyViewValue(array $handles) {
     $value = $this->getFieldValue();
 
-    if (!strlen($value)) {
+    if (!phutil_nonempty_string($value)) {
       return null;
     }