Turbopack: Fix flake in task_statistics unit test (#89981)
If we don't explicitly return a `Vc::cell(())` here, then `<() as dyn
turbo_tasks::vc::default::ValueDefault>::value_default` gets called in a
non-deterministic way.
That's not something we care about testing here, so just add a an
explicit `Vc::cell(())` call to avoid it.
