Previously the plan phase would produce a data diff only if no state was
already present. However, this is a faulty approach because a state will
already be present in the case where the data resource depends on a
managed resource that existed in state during refresh but became
computed during plan, due to a "forces new resource" diff.
Now we will produce a data diff regardless of the presence of the state
when the configuration is computed during the plan phase.
This fixes#6824.
