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.
Our "Parse..." functions all take hcl.Traversal objects rather than strings,
assuming that in most cases we've already parsed a traversal out of some
larger construct (usually a config file) before interpreting it as an
address.
However, there are some situations -- particularly tests -- where being
able to easily parse a string directly is helpful. These new "Parse...Str"
functions all wrap the function of the same name with no "Str" suffix and
first parse the string with the HCL native syntax traversal parser.
As noted in the function doc comments, this should not be used in "real"
code except in exceptional circumstances, since it creates addresses and
diagnostics that do not have useful source location information for
reporting diagnostics.
This function corresponds to terraform.NewInterpolatedVariable, but built
with HCL2 primitives. It accepts a hcl.Traversal, which is what is
returned from the HCL2 API functions to find which variables are
referenced in a given expression.
