BUG: SparseArray.unique with all sparse
xref https://github.com/pandas-dev/pandas/issues/23168

Closes https://github.com/pandas-dev/pandas/issues/19595
Hello @TomAugspurger! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/arrays/sparse.py`](https://github.com/TomAugspurger/pandas/blob/9089d6da849827c7c8a0665016f8c1b3130895cf/pandas/core/arrays/sparse.py) !

 - There are no PEP8 issues in the file [`pandas/tests/arrays/sparse/test_array.py`](https://github.com/TomAugspurger/pandas/blob/9089d6da849827c7c8a0665016f8c1b3130895cf/pandas/tests/arrays/sparse/test_array.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23186?src=pr&el=h1) Report
> Merging [#23186](https://codecov.io/gh/pandas-dev/pandas/pull/23186?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/913f71f74709e4d4814e50f21768b97ce2c6ef8d?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23186/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23186?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23186      +/-   ##
==========================================
- Coverage   92.19%   92.19%   -0.01%     
==========================================
  Files         169      169              
  Lines       50959    50956       -3     
==========================================
- Hits        46980    46977       -3     
  Misses       3979     3979
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <100%> (-0.01%)` | :arrow_down: |
| #single | `42.27% <0%> (-0.02%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23186?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/sparse.py](https://codecov.io/gh/pandas-dev/pandas/pull/23186/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3NwYXJzZS5weQ==) | `92.55% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/numeric.py](https://codecov.io/gh/pandas-dev/pandas/pull/23186/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9udW1lcmljLnB5) | `97.27% <0%> (-0.15%)` | :arrow_down: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/23186/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.55% <0%> (ø)` | :arrow_up: |
| [pandas/core/reshape/merge.py](https://codecov.io/gh/pandas-dev/pandas/pull/23186/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZXJnZS5weQ==) | `93.94% <0%> (+0.04%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23186?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/23186?src=pr&el=footer). Last update [913f71f...abedfe5](https://codecov.io/gh/pandas-dev/pandas/pull/23186?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Ignoring the windows CI failure from yesterday.