fix: handle nil payloads in composite converter

[chris-olszewski]

What was changed

• Now check if the payload is nil before attempting to convert it.
• After chore: generate protobuf rbs #414 we now better typing around our Protobuf messages so we can tighten some of the RBS types. I was hoping this would display the bug as a type error, but it appears the generated RBS types are not honest enough where that happens. Kept the RBS updates as they are still strictly more correct than untyped.

Why?

Go SDK produced workflow and activity completions can be missing the result field if they did not produce a result. This differs from other SDKs (e.g. Ruby) where a payload with binary/null encoding is emitted.

Checklist

1. Closes [Bug] Executing Child Workflow in Go SDK which has no result crashes worker tasks with "NoMethodError: undefined method 'metadata' for nil" #421

2. How was this tested:
   First commit adds failing tests including integration test that uses a Go workflow/activity to verify that Go SDK does return result with a missing result field.

3. Any docs updates needed?
   N/A

[cconstable]

Just curious, why was this needed?