Fix PHP 8.1 "ltrim(null)" exception creating a Nuance Source
Summary:
Passing `null` as the first parameter to `ltrim()` is deprecated since PHP 8.1.
Thus change the optional parameter of the function from null to an empty string.
```
ERROR 8192: ltrim(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/src/applications/nuance/source/NuanceSourceDefinition.php:211]
```
Closes T16000
Test Plan:
* Create a Nuance queue at `/nuance/queue/edit/form/default/`
* Create a Nuance "Web Form" type source at `/nuance/source/edit/form/default/`
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16000
Differential Revision: https://we.phorge.it/D25890