fix null check and shadowed state variable

The tainted state was checked against `cty.NilVal`, however it was
always being set to a null value.

The refreshed state value was being shadowed, and not used in the
following plan.

terraform/node_resource_abstract_instance.go

@@ -905,7 +905,7 @@ func (n *NodeAbstractResourceInstance) plan(
 	// If our prior value was tainted then we actually want this to appear
 	// as a replace change, even though so far we've been treating it as a
 	// create.
-	if action == plans.Create && priorValTainted != cty.NilVal {
+	if action == plans.Create && !priorValTainted.IsNull() {
 		if createBeforeDestroy {
 			action = plans.CreateThenDelete
 		} else {

terraform/node_resource_plan_instance.go

@@ -128,11 +128,12 @@ func (n *NodePlannableResourceInstance) managedResourceExecute(ctx EvalContext)
 
 	// Refresh, maybe
 	if !n.skipRefresh {
-		instanceRefreshState, refreshDiags := n.refresh(ctx, instanceRefreshState)
+		s, refreshDiags := n.refresh(ctx, instanceRefreshState)
 		diags = diags.Append(refreshDiags)
 		if diags.HasErrors() {
 			return diags
 		}
+		instanceRefreshState = s
 
 		diags = diags.Append(n.writeResourceInstanceState(ctx, instanceRefreshState, n.Dependencies, refreshState))
 		if diags.HasErrors() {