Core/Vendors: Support for vendors with empty inventory list

Port From (https://github.com/TrinityCore/TrinityCore/commit/bedc0503578b220b19d23559f670f65d26c8bc87)

Source/Game/Handlers/NPCHandler.cs

@@ -564,6 +564,8 @@ namespace Game
                     break;
             }
 
+            packet.Reason = (byte)(count != 0 ? VendorInventoryReason.None : VendorInventoryReason.Empty);
+
             SendPacket(packet);
         }
     }