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

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/maniphest/xaction/ManiphestTaskPointsTransaction.php:85]
```

Closes T15902

Test Plan: Unclear due to lack of steps to reproduce.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15902

Differential Revision: https://we.phorge.it/D25765

Author: aklapper

src/applications/maniphest/xaction/ManiphestTaskPointsTransaction.php

@@ -82,7 +82,7 @@ public function validateTransactions($object, array $xactions) {
 
     foreach ($xactions as $xaction) {
       $new = $xaction->getNewValue();
-      if (strlen($new) && !is_numeric($new)) {
+      if (phutil_nonempty_string($new) && !is_numeric($new)) {
         $errors[] = $this->newInvalidError(
           pht('Points value must be numeric or empty.'));
         continue;