Although this rarely matters, making it always be nil when empty makes
deep assertions simpler in tests.
This also includes a minor update to the test in the core package that
first encountered this problem, to improve the quality of its output
on failure.
After the refactoring to integrate HCL2 many of the tests were no longer
using correct types, attribute names, etc.
This is a bulk update of all of the tests to make them compile again, with
minimal changes otherwise. Although the tests now compile, many of them
do not yet pass. The tests will be gradually repaired in subsequent
commits, as we continue to complete the refactoring and retrofit work.
