Fix "expected null or a string, got: int" on Jupyter Notebook master
Summary: The value needs to be an integer per specification and per the next check in the code. Previous rPb08c7c761c1078658da02ba68fcb4b0f67912438 was not fully correct. ``` EXCEPTION: (InvalidArgumentException) Call to phutil_nonempty_string() expected null or a string, got: int. at [<arcanist>/src/utils/utils.php:2121] #0 <#2> phutil_nonempty_string(integer) called at [<phorge>/src/applications/files/document/PhabricatorJupyterDocumentEngine.php:326] ``` Fixes T16312 Test Plan: * Go to http://phorge.localhost/file/upload/ * Upload a valid JSON text file: `{"nbformat": 0, "nbformat_minor": 5, "metadata": {}, "cells": []}` * On resulting http://phorge.localhost/F1, select "View Options > View as Jupyter Notebook" Reviewers: O1 Blessed Committers, mainframe98 Reviewed By: O1 Blessed Committers, mainframe98 Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno Maniphest Tasks: T16312 Differential Revision: https://we.phorge.it/D26435