docs: undefined and null are allowed in the each block

Svelte each block now supports undefined and null.

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.

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