TST: Add tests for old issues 2
- [x] closes #14467
- [x] closes #14564
- [x] closes #14773
- [x] closes #15036
- [x] closes #15106
- [x] closes #15239
- [x] closes #15273
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them


This does not yet close #14564, https://github.com/pandas-dev/pandas/issues/14564#issuecomment-609116549?

should maybe change that one to an xref > This does not yet close #14564, #14564 (comment)?

Sorry my comment was a little unclear. Our aggregation ops now support returning `datetime64[ns]` now so returning `NaT` should be expected here as mentioned in the original issue

> 3. Empty frame with datetime values
> 
> df = DataFrame(columns=['a', 'b'], dtype='datetime64')
> df.quantile(0.5, numeric_only=False)
> 
> On 0.18.1 / master, it gives a series of NaNs, where this should be NaTs:
> 
> In [13]: df.quantile(0.5, numeric_only=False)
> Out[13]: 
> a   NaN
> b   NaN
> Name: 0.5, dtype: float64

 thanks @mroeschke 