ENH: Add typing for pandas.core.frame.dropna
This PR adds type hinting for pandas.core.frame.dropna. Additionally, an explicit `return None` was added to the if/else block for in-place changes to meet mypy standards and make the types explicit.

I haven't yet run the tests, because I didn't modify any source code. Once/if the CI builds pass, I'll check it off the list.

- [x] closes #38948
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them