core: fixing a small logic bug in diffList

It’s not enough to only check if no new value is set. It can also be that a new value is set, but contains a variable that cannot be interpolated until a depending resource is created during the apply fase.

I actually found this one as one of the acceptance tests for the AWS ELB resource was failing. It failed with the following error:

```
--- FAIL: TestAccAWSELB_InstanceAttaching (177.83 seconds)
  testing.go:121: Step 1 error: Error applying: aws_elb.bar: diffs didn't match during apply. This is a bug with the resource provider, please report a bug.
FAIL
exit status 1
FAIL  github.com/hashicorp/terraform/builtin/providers/aws  177.882s
```

After a quick look I noticed it was actually a bug in core TF so added the test and made sure all unit tests and AWS acceptance tests are now running successfully.
Does this actually affect the test? It might be simpler to just remove the "internal" part.