BUG: Misc fixes for SparseSeries indexing with MI
- [x] closes #13144
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Fixed following bugs. These makes `MultiIndex` repr work.

```
orig = pd.Series([1, np.nan, np.nan, 3, np.nan], index=idx)
sparse = orig.to_sparse()
sparse['A']    
# IndexError: Out of bounds access
```

```
# the result must have MultiIndex
sparse.loc['A':]
# (A, 0)    1.0
# (A, 1)    NaN
# (B, 0)    NaN
# (C, 0)    3.0
# (C, 1)    NaN
# dtype: float64
# BlockIndex
# Block locations: array([0, 3], dtype=int32)
# Block lengths: array([1, 1], dtype=int32)
```

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13163?src=pr) is **84.12%**

> Merging [#13163](https://codecov.io/gh/pydata/pandas/pull/13163?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **-<.01%**

``` diff
@@             master     #13163   diff @@
==========================================
  Files           138        138          
  Lines         50380      50380          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          42380      42378     -2   
- Misses         8000       8002     +2   
  Partials          0          0          
```
1. File `...das/sparse/series.py` was modified. [more](https://codecov.io/gh/pydata/pandas/commit/eb241028936aa72e21d664c00a802a357c65d0fc/changes?src=pr#70616E6461732F7370617273652F7365726965732E7079) 
   - Misses `+1` 
   - Partials `0` 
   - Hits `-1`

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [4de83d2...eb24102](https://codecov.io/gh/pydata/pandas/compare/4de83d25d751d8ca102867b2d46a5547c01d7248...eb241028936aa72e21d664c00a802a357c65d0fc)
 thanks @sinhrks 
