Adding a test for the change in PR #681

Should have already done that together with that PR…
Merge pull request #685 from svanharmelen/f-add-test-for-diffing-diffs

core: adding a test for the change in PR #681 (diffing diffs)
helper/schema: test for empty state
terraform: Add more tests for cases we felt weren't well covered
terraform: return unknown variable value if nil