TYP: Allow None in Period.strftime
- [ ] closes #xxxx (Replace xxxx with the GitHub issue number)
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Fixes the ASV failures in the first two lines of: https://github.com/pandas-dev/pandas/pull/55179#issue-1899772995

While not familiar with this code, I believe this is the correct resolution because (a) of the ASV `tslibs.period.PeriodUnaryMethods.time_strftime_default` explicitly passes `None` and (b) the `fmt` argument is passed to `period_format` which has the signature `object fmt=None`.
It might be nice to mention in the doc-string what `None` does. @twoertwein - done. Thanks @rhshadrach 