Erm, no? You write f(w: Walrus) -> Walrus and then let the caller handle Walrus|None and Iterable[Walrus] however they wish!
And if someone decides the codebase needs an abstraction over (and therefore specific functions to handle) Iterable[Walrus|None] then you check the weather and suggest they take a break and go for a stroll. (You check the weather to see if you should lend them your brolly.)
What am I missing?