If the delegation (and the default parameters filled in) are part of the function's contract, for example, if the DocString says
doSomething(one, two) is equivalent to
doSomething(one, two, null).
then I refer to these as convenience overloads or convenience methods as they simply save the user some typing but don't do anything the user couldn't have done otherwise.
If the fact that the function simply delegates is not made public, then it is just an arbitrary implementation detail and probably does not deserve a special name.
I've picked this term up from the official Java documentation where it is used quite often.
A related idiom is to have a bunch of overloads for some
doSomething that do nothing more than determining some parameters to then delegate to a
doSomethingImpl. This is something different as the user is not aware of the existence of the
doSomethingImpl method (and couldn't possibly call it). I have heard the
doSomethingImpl be referred to as “worker method” or “workhorse” but those are probably very colloquial terms.