PHP 8.5: Differential: Avoid null array key for revision without history
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."
Do not pass a `null` value in the array down to Arcanist's `array_select_keys()` when there is no `$diff_vs`.
```
ERROR 8192: Using null as the key parameter for array_key_exists() is deprecated, use an empty string instead at [/var/www/html/phorge/arcanist/src/utils/utils.php:634]
#0 array_select_keys(array, array) called at [<phorge>/src/applications/differential/controller/DifferentialRevisionViewController.php:369]
```
Closes T16382
Test Plan: Go to a Differential Revision which only has its first Diff and no History.
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16382
Differential Revision: https://we.phorge.it/D26552