Fix PHP 8.1 "strlen(null)" exception rendering JSON file as Jupyter

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/phorge/src/applications/files/document/PhabricatorJupyterDocumentEngine.php:326]
```

Closes T15951

Test Plan: See T15951

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

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

Maniphest Tasks: T15951

Differential Revision: https://we.phorge.it/D25830

src/applications/files/document/PhabricatorJupyterDocumentEngine.php

@@ -323,7 +323,7 @@ final class PhabricatorJupyterDocumentEngine
     }
 
     $nbformat = idx($data, 'nbformat');
-    if (!strlen($nbformat)) {
+    if (!phutil_nonempty_string($nbformat)) {
       throw new Exception(
         pht(
           'This document is missing an "nbformat" field. Jupyter notebooks '.