Bug in AbstractHolidayCalendar.holidays
...when there are no rules.

```python
from pandas.tseries.holiday import AbstractHolidayCalendar

class ExampleCalendar(AbstractHolidayCalendar):
    pass

cal = ExampleCalendar()
```
```python
In [58]: cal.holidays(pd.Timestamp('01-Jan-2020'), pd.Timestamp('01-Jan-2021'))
Traceback (most recent call last):

  File "<ipython-input-58-022244d4e794>", line 1, in <module>
    cal.holidays(pd.Timestamp('01-Jan-2020'), pd.Timestamp('01-Jan-2021'))

  File "C:\Users\dhirschf\envs\dev\lib\site-packages\pandas\tseries\holiday.py", line 422, in holidays
    self._cache = (start, end, holidays.sort_index())

AttributeError: 'NoneType' object has no attribute 'sort_index'

In [59]: pd.__version__
Out[59]: '0.25.3'
```