COMPAT: should an empty string match a format (or just be NaT)
after #13033

I left this alone, but should an empty string just be a `NaT` (like we do elsewhere), even when a `format`is present? 

```
In [1]:  td = pd.Series(['May 04', 'Jun 02', ''], index=[1, 2, 3])

In [2]: td
Out[2]: 
1    May 04
2    Jun 02
3          
dtype: object

In [3]: pd.to_datetime(td, format='%b %y', errors='coerce')
Out[3]: 
1   2004-05-01
2   2002-06-01
3          NaT
dtype: datetime64[ns]

In [4]: pd.to_datetime(td, format='%b %y', errors='raise')
ValueError: time data '' does not match format '%b %y' (match)
```

related is that we don't coerce empty strings here either (and we have an odd error message).

```
In [1]: pd.to_datetime([1, ''], unit='s', errors='coerce')
Out[1]: DatetimeIndex(['1970-01-01 00:00:01', 'NaT'], dtype='datetime64[ns]', freq=None)

In [2]: pd.to_datetime([1, ''], unit='s')
ValueError: invalid literal for long() with base 10: ''
```
