message: Minor backcompat clean up
  NoClient is included in isSystemMessage in protocol-base since April 2020.
diff: @@ -1390,11 +1390,9 @@ export class DeltaManager
  // All non-system messages are coming from some client, and should have clientId
  // System messages may have no clientId (but some do, like propose, noop, summarize)
  - // Note: NoClient has not been added yet to isSystemMessage (in 0.16.x branch)
  assert(
  message.clientId !== undefined
  - || isSystemMessage(message)
  - || message.type === MessageType.NoClient,
  + || isSystemMessage(message),
  "non-system message have to have clientId",
  );
  