PHP 8.5: Fix null array key in DifferentialParseRenderTestCase

Summary:
Setting null as an array key is deprecated since PHP 8.5 per https://www.php.net/releases/8.5/en.php: "Using null as an array offset or when calling array_key_exists() is now deprecated. Use an empty string instead."

See the same game nine lines lower.

Closes T16497

Test Plan: Run `./bin/arc unit ./src/applications/differential/__tests__/DifferentialParseRenderTestCase.php` on PHP 8.5

Reviewers: O1 Blessed Committers, mainframe98

Reviewed By: O1 Blessed Committers, mainframe98

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16497

Differential Revision: https://we.phorge.it/D26763

src/applications/differential/render/DifferentialChangesetRenderer.php

···
         }
       }
 
-      if (isset($hunk_starts[$ospec['line']])) {
+      if ($ospec['line'] !== null  && isset($hunk_starts[$ospec['line']])) {
         $primitives[] = array(
           'type' => 'no-context',
         );