BUG/PERF: sparse min/max don't densify
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry

This now follows  the structure of other `SparseArray` reductions like `sum` more closely, though there is some added complexity here to avoid taking the `min/max` of an empty array, which would raise when we want to return the corresponding missing value instead.

Benchmarks:
```
       before           after         ratio
     [5deec13d]       [f3eea313]
     <master>         <perf_sparse_min_max>
-      4.05±0.3ms       11.6±0.3μs     0.00  sparse.MinMax.time_min_max('max', 0.0)
-      4.49±0.4ms       11.6±0.2μs     0.00  sparse.MinMax.time_min_max('min', 0.0)
-      4.62±0.7ms       10.6±0.2μs     0.00  sparse.MinMax.time_min_max('max', nan)
-      5.37±0.7ms       10.3±0.3μs     0.00  sparse.MinMax.time_min_max('min', nan)
```