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