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.