Transition flag from null to undefined (#831)

## Description:

Replace `null` with `undefined` for player flags.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

---------

Co-authored-by: Scott Anderson <662325+scottanderson@users.noreply.github.com>

src/server/Client.ts

@@ -18,6 +18,6 @@ export class Client {
     public readonly ip: string,
     public readonly username: string,
     public readonly ws: WebSocket,
-    public readonly flag: string | null,
+    public readonly flag: string | undefined,
   ) {}
 }

src/server/GameServer.ts

@@ -296,7 +296,7 @@ export class GameServer {
         clientID: c.clientID,
         flag: c.flag,
       })),
-    });
+    } satisfies GameStartInfo);
 
     this.endTurnIntervalID = setInterval(
       () => this.endTurn(),