BUG: Fix creating DatetimeIndex with BusinessHour Frequency
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:
```python
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

- [x] closes #49835 
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Great! Thanks @DavidKleindienst 