BUG: PeriodIndex[B].to_timestamp drops missing days
### 

- [X] I have checked that this issue has not already been reported.

- [X] I have confirmed this bug exists on the [latest version](https://pandas.pydata.org/docs/whatsnew/index.html) of pandas.

- [X] I have confirmed this bug exists on the master branch of pandas.


### Reproducible Example

```python
dti = pd.date_range("2021-10-18", periods=9, freq="B")
pi = dti.to_period()

roundtrip = pi[::2].to_timestamp()  # should match dti[::2]  (except for roundtrip.freq)

>>> roundtrip
DatetimeIndex(['2021-10-18', '2021-10-19', '2021-10-20', '2021-10-21',
               '2021-10-22'],
              dtype='datetime64[ns]', freq='D')

>>> roundtrip - dti[::2]  # should be timedelta-zeros
TimedeltaIndex(['0 days', '-1 days', '-2 days', '-5 days', '-6 days'], dtype='timedelta64[ns]', freq=None)
```



### Issue Description

PeriodArray.to_timestamp is returning incorrect results.  Haven't checked if Period is affected, or other freqs.

### Expected Behavior

`assert (roundtrip == dti[::2]).all()`

### Installed Versions

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Replace this line with the output of pd.show_versions()

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