[Game Server] Null reference risks in MatchManager, ReadySystem, MatchUtility

[Flegma]

Summary

Several locations access objects without null checks, risking NullReferenceException or KeyNotFoundException.

Findings

• MatchManager.cs — _matchData checked for null then accessed in lambda. Race condition if it becomes null between check and access.
• ReadySystem.cs — _readyPlayers[player.UserId.Value] without checking key exists. KeyNotFoundException on reconnect.
• MatchUtility.cs — tag == null check on non-nullable string. Should use string.IsNullOrEmpty().

Impact

Server crashes when players reconnect or during race conditions.

Suggested Fix

Use TryGetValue for dictionary access. Store _matchData in local variable before lambda. Use string.IsNullOrEmpty().