BUG: repr SparseDataFrame after setting a value
 - [x] closes #15488
 - [x] tests added / passed
 - [X] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15489?src=pr&el=h1) Report
> Merging [#15489](https://codecov.io/gh/pandas-dev/pandas/pull/15489?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d80275dfaa6a8ad50bc49dbaef9eacd5509008dc?src=pr&el=desc) will **decrease** coverage by `-0.01%`.
> The diff coverage is `n/a`.


```diff
@@            Coverage Diff             @@
##           master   #15489      +/-   ##
==========================================
- Coverage   90.36%   90.36%   -0.01%     
==========================================
  Files         136      136              
  Lines       49553    49551       -2     
==========================================
- Hits        44781    44779       -2     
  Misses       4772     4772
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15489?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/formats/format.py](https://codecov.io/gh/pandas-dev/pandas/compare/d80275dfaa6a8ad50bc49dbaef9eacd5509008dc...2dc145c9ccc46ab5335baf53ffc5950b92a47a17?src=pr&el=tree#diff-cGFuZGFzL2Zvcm1hdHMvZm9ybWF0LnB5) | `95.31% <ø> (-0.01%)` | :x: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15489?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/15489?src=pr&el=footer). Last update [d80275d...2dc145c](https://codecov.io/gh/pandas-dev/pandas/compare/d80275dfaa6a8ad50bc49dbaef9eacd5509008dc...2dc145c9ccc46ab5335baf53ffc5950b92a47a17?el=footer&src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). ok, lgtm. ping on green. It's green. thanks @kernc 

keep em coming!