[TS] Narrow types for Show utility (#701)

* Narrow types for Show utility

* TIL that 0n is also falsy

I don't know if I like that. 0n feels more object-y than 0 to me

* Use NonNullable<T>. That's what TS type narrowing does by default.

Downside: It won't correctly handle literal values like "hello" vs "", but oh well.

* Add changeset

---------

Co-authored-by: Jovi De Croock <[email protected]>

Author: calebeby

.changeset/plenty-bikes-fold.md

@@ -0,0 +1,5 @@
+---
+"@preact/signals": patch
+---
+
+Narrow types for Show utility, the callback is truthy by design

packages/preact/utils/src/index.ts

@@ -6,7 +6,7 @@ import { useMemo } from "preact/hooks";
 interface ShowProps<T = boolean> {
 	when: Signal<T> | ReadonlySignal<T>;
 	fallback?: JSX.Element;
-	children: JSX.Element | ((value: T) => JSX.Element);
+	children: JSX.Element | ((value: NonNullable<T>) => JSX.Element);
 }
 
 export function Show<T = boolean>(props: ShowProps<T>): JSX.Element | null {