API: change default for sep in str.cat (in docstring)
In the course of #22725, @WillAyd mentioned (about the `sep=None` in `Series.str.cat` being immediately overwritten by `sep = '' if sep is None else sep`):
> Not anything you need to change in this PR but if we just overwrite `None` here it would probably make more sense to change the default function signature to simply be `''`

This changes the default accordingly. Not sure this even needs a deprecation cycle? It's a pretty trivial change, and I doubt there are people relying on `sep=None` explicitly.
