PHP 8.5: Avoid PhpStan warning about possible null array key in PhabricatorApplicationTransactionView
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."
Test Plan:
Run static code analysis:
```
/src/applications/transactions/view/PhabricatorApplicationTransactionView.php:372 Possibly invalid array key type int|string|null.
```
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D26679