background.ts: Introduce types for redux initialState

ts/messages/MessageSendState.ts

@@ -1,4 +1,4 @@
-// Copyright 2021 Signal Messenger, LLC
+// Copyright 2021-2022 Signal Messenger, LLC
 // SPDX-License-Identifier: AGPL-3.0-only
 
 import { makeEnumParser } from '../util/enum';
@@ -150,10 +150,12 @@ export const someSendStatus = (
 
 export const isMessageJustForMe = (
   sendStateByConversationId: undefined | Readonly<SendStateByConversationId>,
-  ourConversationId: string
+  ourConversationId: string | undefined
 ): boolean => {
   const conversationIds = Object.keys(sendStateByConversationId || {});
-  return (
-    conversationIds.length === 1 && conversationIds[0] === ourConversationId
+  return Boolean(
+    ourConversationId &&
+      conversationIds.length === 1 &&
+      conversationIds[0] === ourConversationId
   );
 };