storage: fix potential kafka sink bug

[aljoscha]

Tips for reviewer

The comments should explain it all, but please yell at me if they don't.

Also: I should probably downgrade the logging to trace! before merging, but these helped in debugging another bug (https://github.com/MaterializeInc/materialize/issues/14754)

Checklist

• [ ] This PR has adequate test coverage / QA involvement has been duly considered.

• [ ] This PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way) and therefore is tagged with a T-protobuf label.

• [ ] This PR includes the following user-facing behavior changes: