This PR replaces our use of cty.NilVal with cty.NullVal. The included ternary-mismatch.json resulted in a panic from this line:

bucket = var.foo ? {"foo": "bar"} : aws_s3_bucket.foo.bucket

because the HCL library's error handling for mismatched conditional return types attempts to call a FriendlyName() method on each return type. cty.NilVal.Type() returns a native nil value, which results in a panic.