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
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16892?src=pr&el=h1) Report
> Merging [#16892](https://codecov.io/gh/pandas-dev/pandas/pull/16892?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a9421af1aac906cc38d025ed5db4a2b55cb8b9bc?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16892/graphs/tree.svg?width=650&height=150&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16892?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16892      +/-   ##
==========================================
+ Coverage   90.96%   90.97%   +<.01%     
==========================================
  Files         161      161              
  Lines       49301    49299       -2     
==========================================
+ Hits        44849    44850       +1     
+ Misses       4452     4449       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.74% <100%> (+0.02%)` | :arrow_up: |
| #single | `40.19% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16892?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/sparse/array.py](https://codecov.io/gh/pandas-dev/pandas/pull/16892?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3BhcnNlL2FycmF5LnB5) | `91.42% <100%> (-0.05%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16892?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16892?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.71% <0%> (-0.1%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/16892?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.05% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16892?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16892?src=pr&el=footer). Last update [a9421af...c1cd33e](https://codecov.io/gh/pandas-dev/pandas/pull/16892?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 can you rebase and update thanks!