HDFStore: Fix empty result of keys() method on non-pandas hdf5 file
First try to get pandas style tables, if there are none, return the list of non-pandas (hdf5 native) tables in the file (if any)

- [x] closes #29916
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
