The cty change here fixes a panic situation when cty.Path.Apply is given
a null value, making it now correctly return an error.
However, the HCL2 change includes an alternative to cty.Path.Apply that
uses HCL-level rules rather than cty-level rules, so the result behaves
like an HCL expression would. Most uses of cty.Path.Apply ought to use
hcl.ApplyPath instead, to ensure that the behavior is consistent with what
users expect in the main language.
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