add typehint for None pct_change
- [x ] closes #51269 
- [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)

I've added a typehint for None. It looks like supplying `None` as an argument to `fill_method` is tested for dataframes under `pandas/tests/series/methods`.

Let me know if another test should be written, e.g for dataframes.
> It looks like supplying None as an argument to fill_method is tested for dataframes under pandas/tests/series/methods.

you're right, thanks

agree that it would be good to document what passing `None` does Would you like me to write docs on this branch / PR, or should that be done on another?