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

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 #1 ($string) of type string is deprecated at [/var/www/html/phorge/arcanist/src/parser/xhpast/__tests__/PHPASTParserTestCase.php:85]
```

Refs T15926

Test Plan: Either read the code, or probably run something like `arc unit` on D25797?

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15926

Differential Revision: https://we.phorge.it/D25800

src/parser/xhpast/__tests__/PHPASTParserTestCase.php

@@ -82,7 +82,7 @@ final class PHPASTParserTestCase extends PhutilTestCase {
       case 'pass':
         $this->assertEqual(0, $err, pht('Exit code for "%s".', $name));
 
-        if (!strlen($expect)) {
+        if (!phutil_nonempty_string($expect)) {
           // If there's no "expect" data in the test case, that's OK.
           break;
         }