Fix protobuf syntax that is causing an error in protobufjs (#589)

**What changed?**

- Fix a double semicolon introduced in #579.

**Why?**

- The double semicolon sequence is causing a compilation error in
`protobufjs`, even though this is technically valid according to [the
official Protobuf 3
spec](https://protobuf.dev/reference/protobuf/proto3-spec/#emptystatement).

This is a bug in `protobufjs`, for which there's been [an open issue
ticket](protobufjs/protobuf.js#1322) since
2019, but it was never acknowledged, and a PR fixing this has been
ignored for more than a year.

Given that there's simply no reason to use a double semicolon sequence
anyway, it appears preferable to simply avoid that in our proto
definitions.

Author: mjameswh