decision(firestore-translate-text): null input routes to translateSingle instead of throwing

[cabljac]

The extension checks typeof input === "object", so a null input routes into translateMultiple and throws in Object.entries(null) (functions/src/translate/translateMultiple.ts:35). The kit adds && input !== null (kits/firestore-translate-text/src/translate/translateDocument.ts:44), which routes null to translateSingle instead. Only reachable on the update path, since create is guarded by if (input).

#3109 (#3106) added a test that pins the kit's routing (tests/translate-document.test.ts:116), so the divergence is now deliberate in code but not yet ruled.

Decide one of:

• Keep the kit's guard: the extension threw an unhandled TypeError here, so this is a fix, not a divergence. Record it in the Notes entries (docs(kits): write the Notes entries for accepted divergences #3035).
• Parity: drop the guard and let null fail the same way. Unlikely to be wanted, since the extension behaviour is a crash.

Parity ledger: #2974, firestore-translate-text §4a.