firestore-translate-text: a null input on update errors and leaves stale translations instead of clearing them, in both extension and kit

Setting the input field to `null` on an existing document is the one "no text here" value that neither clears the translations nor gets skipped. It errors out and leaves the previous translations in place, in both the extension and the kit.

`handleUpdateDocument` clears the output field when the new input cannot be translated:

```ts
if (typeof inputAfter !== "string" && typeof inputAfter !== "object") {
await updateTranslations(after, FieldValue.delete());
return;
}
```

`typeof null` is `"object"`, so `null` passes this check and falls through to `translateDocument`. Removing the field (`undefined`) or setting a number both hit the branch and clear `translated`; `null` does not.

**Extension** (`firestore-translate-text/functions/src/index.ts:236`, `translate/translateDocument.ts:66`): `null` routes into `translateMultiple`, `Object.entries(null)` throws `TypeError`, the handler catch logs it and records an error event. Nothing is written, so the stale `translated` map stays on the document.

**Kit** (`kits/firestore-translate-text/src/handlers.ts:147`): same check, same fall-through. Routing parity with the extension is restored under #3142, so the failure is identical.

Only reachable on update; create is guarded by `if (input)`.

Proposed fix, one change across both codebases so they keep producing the same output for the same document: treat `null` as removed input in the delete branch.

```ts
if (
inputAfter === null ||
(typeof inputAfter !== "string" && typeof inputAfter !== "object")
) {
```

Plus one handler test: before `input: "hello"`, after `input: null`, expect `updateTranslations` called with `FieldValue.delete()` and no translation call. Behaviour change for the extension too (error becomes clear), so it needs a CHANGELOG entry on both sides.

Split out of #3142, where the ruling was parity for the routing. Parity ledger: #2974, firestore-translate-text §4a.

Start with handleUpdateDocument in firestore-translate-text/functions/src/index.ts:236 and the corresponding handler in kits/firestore-translate-text/src/handlers.ts:147; inspect translate/translateDocument.ts:66 to confirm the null path. Add the handler test described in the issue, checking deletion and no translation call, then add CHANGELOG entries for both codebases. Done means null updates clear translated without an error and extension and kit retain identical behavior.

Written by the indexing model from the issue text.