fix(core): Fixes buy packet (#453)

- [X] Fixes buy packet
- [X] Changes from null to empty string in gump initial text

Projects/Server/Network/NetState/NetState.cs

@@ -606,7 +606,7 @@ namespace Server.Network
 
         public bool HandleReceive()
         {
-            if (Connection == null)
+            if (Connection == null || !_running)
             {
                 return false;
             }