PHP 8.5: DifferentialChangesetTwoUpRenderer: Do not pass null array key

Summary:
See twenty lines below how to do it right.

```
ERROR 8192: Using null as an array offset is deprecated, use an empty string instead at [/var/www/html/phorge/phorge/src/applications/differential/render/DifferentialChangesetTwoUpRenderer.php:261]
```

Closes T16362

Test Plan: Go to a Differential revision like http://phorge.localhost/D1

Reviewers: O1 Blessed Committers, mainframe98

Reviewed By: O1 Blessed Committers, mainframe98

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16362

Differential Revision: https://we.phorge.it/D26531

src/applications/differential/render/DifferentialChangesetTwoUpRenderer.php

···
         }
       }
 
-      if (isset($hunk_starts[$o_num])) {
+      if ($o_num && isset($hunk_starts[$o_num])) {
         $html[] = $context_not_available;
       }