Binary testing: omit test cleanup when state is empty

[kmoe]

Fixes #347

The legacy test framework does not attempt a post-test destroy step when there is no state: https://github.com/hashicorp/terraform-plugin-sdk/blob/master/helper/resource/testing.go#L701

This was missed out of the new framework, with the consequence that successful tests that do not create resources would error during cleanup.

We add this check back, in new binary testing style - the nuance here being that our state may well contain modules but no resources.

Again, I'll raise a separate PR to the version2 branch after review.

[appilon]

Curious is it possible to have a test validate this behavior? If effort is too high for that at this time this is ready to merge

[ghost]

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