BUG: Made SparseDataFrame.fillna() fill all NaNs
A continuation of https://github.com/pandas-dev/pandas/pull/16178

 - [x] fixes https://github.com/pandas-dev/pandas/issues/16112, closes https://github.com/pandas-dev/pandas/pull/16178
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff`` (On Windows, ``git diff upstream/master -u -- "*.py" | flake8 --diff`` might work as an alternative.)
 - [x] whatsnew entry