createWritableMemo() doesn't work with undefined

[astoilkov]

Describe the bug

Passing undefined to the createWritableMemo() setter doesn't update the memo toundefined.

import { createWritableMemo } from "@solid-primitives/memo";

const [count, setCount] = createSignal(1);
const [result, setResult] = createWritableMemo(() => count() * 2);
result(); // => 2
setResult(undefined); // overwrites calculation result
result(); // => 2 ❌ incorrect it should be undefined

Minimal Reproduction Link

https://playground.solidjs.com/anonymous/76de3e3d-d0e1-457f-8d50-1cebfead6c1c