BUG: Sparse creation with object dtype may raise TypeError
closes #11633 
closes #11856

Added basic tests for `groupby` and `pivot_table`.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13201?src=pr) is **84.15%**

> Merging [#13201](https://codecov.io/gh/pydata/pandas/pull/13201?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13201   diff @@
==========================================
  Files           138        138          
  Lines         50388      50394     +6   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42386      42407    +21   
+ Misses         8002       7987    -15   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [4e4a7d9...443b47e](https://codecov.io/gh/pydata/pandas/compare/4e4a7d9ae5b3c05b21dc0a0f3c12648236ec586e...443b47e83be2191477f6a8530e06001f5da69ce0)
 thanks @sinhrks very nice as usual

side-note (nothing to do with this PR I think, but should investigate at some point).

```
nosetests pandas/sparse
....
/Users/jreback/pandas/pandas/sparse/array.py:52: FutureWarning: comparison to `None` will result in an elementwise object comparison in the future.
  return _wrap_result(name, op(self.sp_values, other),
```
