BUG: Fix creating DatetimeIndex with BusinessHour Frequency

[DavidKleindienst]

DatetimeIndex creating with BusinessHour Frequency would fail whenever the DatetimeIndex did not include a Timestamp that was also an opening time.

The cause of the issue was a bug where adding/subtracting 0 * BusinessHour gave wrong results:

import pandas as pd
import datetime as dt
dt.datetime(2020,1,1,10) -0 * pd.offsets.BusinessHour()

returns Timestamp('2020-01-02 09:00:00') rather than the expected Timestamp('2020-01-01 10:00:00')

This PR fixes the problem and adds appropriate assertions to the unit-tests

• closes BUG: DatetimeIndex creation with BusinessHour frequency fails when first hour of the day is not included as Timestamp #49835
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

[mroeschke]

Suggested change

	- Fixed bug in :class:`BusinessHour` that could cause creation of ``DatetimeIndex`` to fail (:issue:`#49835`)
	- Fixed bug in :class:`BusinessHour` that could cause creation of :class:`DatetimeIndex` to fail (:issue:`49835`)

1. Could you move this to v2.0.0.rst?
2. Could you make this description more specific too?

[DavidKleindienst]

I've written it like this now, is that ok?
- Bug in :class:`BusinessHour` would cause creation of :class:`DatetimeIndex` to fail when no opening hour was included in the index (:issue:`#49835`)

Thanks for reviewing!

[mroeschke]

Great! Thanks @DavidKleindienst