PHP 8.5: Make getCanonicalValue() handle passed null values in PhabricatorSelectEditField
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."
```
ERROR 8192: Using null as an array offset is deprecated, use an empty string instead at [/var/www/html/phorge/phorge/src/applications/transactions/editfield/PhabricatorSelectEditField.php:64]
```
Closes T16366
Test Plan: Go to http://phorge.localhost/settings/panel/datetime/
Reviewers: O1 Blessed Committers, avivey
Reviewed By: O1 Blessed Committers, avivey
Subscribers: avivey, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16366
Differential Revision: https://we.phorge.it/D26537