Fix PHP 8.1 PhabricatorEditorURIEngine::newForViewer() trim(NULL) error

Summary:
Under PHP 8.1, PhabricatorEditorURIEngine::newForViewer() is throwing a trim(NULL) error when trying to view a diff.
This is because it tries to apply string operations to a user setting which will be null by default.

Fixes T15518

Test Plan:
Unit test added -

arc unit

Or just view a diff. Eg:
https://my.phorge.site/D1234

Reviewers: O1 Blessed Committers, valerio.bozzolan, avivey

Reviewed By: O1 Blessed Committers, valerio.bozzolan, avivey

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15518

Differential Revision: https://we.phorge.it/D25324