Conduit: Fix PHP 8.1 "trim(null)" exception in conduit.getcertificate
Summary:
Passing null to `trim()` is deprecated since PHP 8.1.
```
ERROR 8192: trim(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/conduit/method/ConduitGetCertificateConduitAPIMethod.php:55]
```
Closes T16423
Test Plan:
* PHP 8.1+
* Go to http://phorge.localhost/conduit/method/conduit.getcertificate/
* Press the "Call Method" button
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16423
Differential Revision: https://we.phorge.it/D26634