This has been against the documented typing of the functions for some
time, but some scheduling methods have mistakenly been passing `None`.
There is no need to support `None` here; the empty tuple `()` is equally
immutable and a CPython singleton, so there are neither mutability nor
memory benefits to supporting both, and removing `None` as an input is a
simple way to remove a(n admittedly cheap) branch from the appender
methods.