Don't crash when populating NewExtra.

When populating NewExtra, we iterate over attributes and inspect their
NewExtra property. In the case of null attributes, this results in a
crash, as we're dereferencing a nil pointer.

I'm still not clear on why this is happening, where it's coming from, or
what it means. The current understanding is it has something to do with
usage of CustomizeDiff. I'm not sure why yet. But this commit at least
surfaces which attribute was null, by logging it, and prevents the
crash, even if it doesn't resolve the problem.

helper/schema/grpc_provider.go

@@ -796,6 +796,10 @@ func (s *GRPCProviderServer) PlanResourceChange(ctx context.Context, req *tfprot
 	newExtra := map[string]interface{}{}
 
 	for k, v := range diff.Attributes {
+		if v == nil {
+			log.Printf("[WARN] Field %q was null, not modifying its NewExtra", k)
+			continue
+		}
 		if v.NewExtra != nil {
 			newExtra[k] = v.NewExtra
 		}