docs: update #each to reflect support for null and undefined (#17339)

* docs: update #each to reflect support for null and undefined

* update

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

documentation/docs/03-template-syntax/03-each.md

@@ -12,7 +12,9 @@ title: {#each ...}
 {#each expression as name, index}...{/each}
 ```
 
-Iterating over values can be done with an each block. The values in question can be arrays, array-like objects (i.e. anything with a `length` property), or iterables like `Map` and `Set` — in other words, anything that can be used with `Array.from`.
+Iterating over values can be done with an each block. The values in question can be arrays, array-like objects (i.e. anything with a `length` property), or iterables like `Map` and `Set`— in other words, anything that can be used with `Array.from`.
+
+If the value is `null` or `undefined`, it is treated the same as an empty array (which will cause [else blocks](#Else-blocks) to be rendered, where applicable).
 
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