API: make min/max on empty datetime df consistent with datetime serie…
…s (#33704)

- [x] closes #33704
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Fixes the following issue:
```python
import pandas as pd
df = pd.DataFrame(dict(x=pd.to_datetime([])))
df.max()
```
throws a `ValueError` but 
```python
pd.Series(pd.to_datetime([])).max()
```
results in `NaT`.
With this change, calling an DataFrame on en empty `pd.to_datetime([])`, results in
```
In[1]: df = pd.DataFrame(dict(x=pd.to_datetime([])))
In[2]: df.max()
Out[5]: 
x   NaT
dtype: datetime64[ns]
```

Hello @CloseChoice! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-05-07 22:16:22 UTC > pls add a whatsnew note as well, 1.1 bug fixes datetimelike section

done thanks @CloseChoice 