Port upstream Muya history identity op guard

[Renakoni]

Summary

Ports upstream marktext/marktext PR #4838 into the Android Muya copy.

Upstream source: marktext/marktext#4838

This is a follow-up hardening PR for upstream #4815, so this Android PR is stacked on Android PR #60 (sync/upstream-4815-identity-flush). json-change can still carry the identity operation (null) through non-deferred paths such as Editor.updateContents(null). History reads op.length, so it must treat null as a no-op rather than recording or transforming it.

Changes

• Changes History's json-change listener to accept Nullable<JSONOpList>.
• Ignores null identity ops before _record() / _transform().
• Adds regression coverage for updateContents(null) and the compose-to-null deferred flush path.

Verification

• pnpm test -- src/history/__tests__/identityOpJsonChange.spec.ts src/state/__tests__/flushPendingOps.spec.ts from third_party/muya passed: 7 tests.
• pnpm exec eslint src/history/index.ts src/history/__tests__/identityOpJsonChange.spec.ts --no-ignore --max-warnings 0 from third_party/muya passed.
• pnpm typecheck from repo root passed.

Audit Decision

This upstream PR is Android-relevant because it hardens Muya's state/history path for identity operations, including IME-like edit cancellation cases. Android heavily depends on IME behavior in the WebView editor, and this PR closes the consumer-side crash even if another emitter forwards null in the future.