Use fresh state for list item dataloader fibers

[rmosolgo]

This runtime_state object was previously being passed into a @dataloader.append_job block so it could be reused. But if that object was reused, it wouldn't be written to Fiber.current[...] and therefore wouldn't be available to Context#current_path. (And if it was available there, it would have been buggy, since the same state was being shared between fibers.)

So pass nil instead of the existing state and allow the method to fall back to a new state object when one isn't pass (as done elsewhere in this file).

Fixes #5595