Correctly handle empty tuple index when unpacked (#21545)

Fixes https://github.com/python/mypy/issues/18390
Closes https://github.com/python/mypy/pull/18395

This is a more targeted fix than the original PR. Note we don't need to
worry about other places with `flatten_nested_tuples()`, since the issue
is only specific to (early) `UnboundType`.