Issue #3610015: Fix a fatal in the settle task's config form when there is no user input

Building the SettleTask settings form outside a request (the modeler rendering it) passes a form state whose getUserInput() is null. The payable-resolver picker in PayableResolverConfigTrait read that raw input to keep a live AJAX selection, which fatally failed: NestedArray::getValue() requires an array and takes it by reference. This defaults the input to an array held in a local variable.

Regression test added: the settle task config form builds with no user input. Introduced in #3610001.