BUG: GH13629 Binned groupby median function calculates median on empt…
- [ x] closes #13629
- [ x] tests added / passed
- [ x] passes `git diff upstream/master | flake8 --diff`
- [x ] whatsnew entry

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14225?src=pr) is 85.24% (diff: 100%)

> Merging [#14225](https://codecov.io/gh/pydata/pandas/pull/14225?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14225   diff @@
==========================================
  Files           140        140          
  Lines         50559      50560     +1   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43099      43101     +2   
+ Misses         7460       7459     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [e8357a1...42affd5](https://codecov.io/gh/pydata/pandas/compare/e8357a15cd61ff698cbd3d57904133c586a8ed8b...42affd50b9dc728d5b8943cafe475327142ac5a7?src=pr)
 lgtm, @jreback?
 lgtm

merge away
