Pass unmatched calls through when branch has no fallback

[soroush5]

Fixes #940. (The repro in draft #939 should now pass with this change.)

composer.on("sendMessage") crashed with TypeError: Cannot read properties of undefined (reading 'transformer') for any nonmatching method. Root cause is in branch(): filter() calls it without a fallback, and it built new TransformerComposer(falseTransformer) with falseTransformer === undefined — a one-element chain whose flatten step blows up on undefined.

The fix uses an empty composer (identity pass-through) when no fallback is given, so unmatched calls flow to previous untouched.

How I verified: new test/transform.test.ts — a nonmatching getMe reaches previous exactly once (fails before with the exact reported TypeError, passes after), plus a matching-method test proving scoped transformers still run. Neighbors (composer, filter) green: 5 files / 78 steps, deno lint + deno fmt --check clean.

[soroush5]

You're right, thanks for pointing it out — #956 was already open and covers this. Closing mine to keep the review queue clean; happy to help test or review #956 instead.