docs: `undefined` and `null` are allowed in the `each` block

[hyunbinseo]

Describe the bug

Svelte each block now supports undefined and null.

• fix: allow undefined and null values for #each in Svelte 5 language-tools#2863

The docs should be updated to match this behavior.

Current explanation is incorrect:

in other words, anything that can be used with Array.from.

Array.from(undefined); // Uncaught TypeError: can't access property Symbol.iterator, items is undefined
Array.from(null); // Uncaught TypeError

Stating this in the docs will avoid LLMs from writing ?? [] inside the each loop.

Reproduction

Logs

System Info

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Severity

annoyance