Fix crosstab(aggfunc='skew') bug
Fix `IndexError` in `crosstab` with `aggfunc=‘skew’`.

* Check if table is empty before accessing last row in `_normalize` function in `pandas/core/reshape/pivot.py`.
* Skip normalization and return empty table if it is.
* Add test case in `pandas/tests/reshape/test_crosstab.py` for `aggfunc=‘skew’`.
* Ensure test case doesn’t raise `IndexError`.


Thanks for the pull request, but it appears to have gone stale. If interested in continuing, please merge in the main branch, address any review comments and/or failing tests, and we can reopen.