It seems intuitive for merge() to return an empty object when the inputs
are null (or omitted entirely) - returning an error would likely break
many terraform configurations, and returning an empty object rather than
null seems more consistent with the documented behavior (though it is
not explicit!).

I dug through the commit history and compared terraform's documentation,
post pre- and post-hcl2, and did not find any clues as to why it was
returning null rather than an empty object. I feel like this is an
acceptable breaking change, but am certainly open to other opinions.