Core/Mail: Reorder mail attachment checks to be able to report nonempty bag error to client.

Port From (https://github.com/TrinityCore/TrinityCore/commit/7b48268e2cfc7fddeba8fe55721aaab4234cc152)

Source/Game/Handlers/MailHandler.cs

@@ -186,6 +186,13 @@ namespace Game
                         return;
                     }
 
+                    // handle empty bag before CanBeTraded, since that func already has that check
+                    if (item.IsNotEmptyBag())
+                    {
+                        player.SendMailResult(0, MailResponseType.Send, MailResponseResult.EquipError, InventoryResult.DestroyNonemptyBag);
+                        return;
+                    }
+
                     if (!item.CanBeTraded(true))
                     {
                         player.SendMailResult(0, MailResponseType.Send, MailResponseResult.EquipError, InventoryResult.MailBoundItem);
@@ -213,12 +220,6 @@ namespace Game
                         return;
                     }
 
-                    if (item.IsNotEmptyBag())
-                    {
-                        player.SendMailResult(0, MailResponseType.Send, MailResponseResult.EquipError, InventoryResult.DestroyNonemptyBag);
-                        return;
-                    }
-
                     items.Add(item);
                 }