TST: Add test to verify 'dropna' behaviour on SparseArray
verify that calling `dropna` on a pd.SparseArray does not inadvertently drop non-na records on both DataFrames and the SparseArray itself.

- [x] closes #28287
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry (not applicable?)

Thanks, incorporated the feedback! > The test you added looks good, but I think it would be nice to directly include a test for "invert" as well. See the comment from the original issues: [#28287 (comment)](https://github.com/pandas-dev/pandas/issues/28287#issuecomment-529657096)

Isn't that already covered by https://github.com/pandas-dev/pandas/blob/72aed3e3052ef92005408a40c34425cd1f09fadd/pandas/tests/arrays/sparse/test_arithmetics.py#L466-L480 ?
 > Isn't that already covered by 

That indeed looks to be the case, thanks for checking! (failures are all the html failures) Thanks!