CLN: private funcs in concat.py
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Refactor/cleanup ``_get_empty_dtype_and_na`` in ``pandas/core/internals/concat.py``
Extract functions, add typing.

Thanks @ivanovmg 