PHP 8.5: Avoid null array key creating Passphrase Credential without type
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."
```
[2026-01-06 23:02:59] ERROR 8192: Using null as an array offset is deprecated, use an empty string instead at [/var/www/html/phorge/phorge/src/applications/passphrase/controller/PassphraseCredentialCreateController.php:17]
```
Closes T16447
Test Plan: On PHP 8.5, go to http://phorge.localhost/passphrase/create/ and press "Continue" without selecting a credential type.
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16447
Differential Revision: https://we.phorge.it/D26663