interp: unset positional parameters are not considered unset

[koonix]

This behavior is at least inconsistent with bash and dash.

echo ${1?} # should complain, but does not
echo ${1-hello} # should echo hello, but does not
set -u
echo $1 # should complain, but does not
echo success.

https://go.dev/play/p/2JSDtp-5w7M