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)
```
