Core/Vendors: allow players to interact with vendors that do not have any items to sell

src/server/game/Entities/Creature/Creature.h

@@ -53,6 +53,7 @@ typedef std::list<VendorItemCount> VendorItemCounts;
 #define CREATURE_Z_ATTACK_RANGE 3
 
 #define MAX_VENDOR_ITEMS 150                                // Limitation in 4.x.x item count in SMSG_VENDOR_INVENTORY
+static constexpr uint8 VENDOR_INVENTORY_REASON_INVENTORY_EMPTY = 1;
 
 //used for handling non-repeatable random texts
 typedef std::vector<uint8> CreatureTextRepeatIds;

src/server/game/Entities/Player/Player.cpp

@@ -13814,16 +13814,6 @@ void Player::PrepareGossipMenu(WorldObject* source, uint32 menuId /*= 0*/, bool
                     if (!isDead())
                         canTalk = false;
                     break;
-                case GOSSIP_OPTION_VENDOR:
-                {
-                    VendorItemData const* vendorItems = creature->GetVendorItems();
-                    if (!vendorItems || vendorItems->Empty())
-                    {
-                        TC_LOG_ERROR("sql.sql", "Creature %s (Entry: %u GUID: %u DB GUID: %u) has UNIT_NPC_FLAG_VENDOR set, but has an empty trading item list.", creature->GetName().c_str(), creature->GetEntry(), creature->GetGUID().GetCounter(), creature->GetSpawnId());
-                        canTalk = false;
-                    }
-                    break;
-                }
                 case GOSSIP_OPTION_LEARNDUALSPEC:
                 case GOSSIP_OPTION_DUALSPEC_INFO:
                     if (!(GetSpecsCount() == 1 && creature->CanResetTalents(this) && !(getLevel() < sWorld->getIntConfig(CONFIG_MIN_DUALSPEC_LEVEL))))
@@ -13864,6 +13854,7 @@ void Player::PrepareGossipMenu(WorldObject* source, uint32 menuId /*= 0*/, bool
                 case GOSSIP_OPTION_PETITIONER:
                 case GOSSIP_OPTION_TABARDDESIGNER:
                 case GOSSIP_OPTION_AUCTIONEER:
+                case GOSSIP_OPTION_VENDOR:
                     break;                                  // no checks
                 case GOSSIP_OPTION_OUTDOORPVP:
                     if (!sOutdoorPvPMgr->CanTalkTo(this, creature, itr->second))

src/server/game/Handlers/ItemHandler.cpp

@@ -710,6 +710,9 @@ void WorldSession::SendListInventory(ObjectGuid vendorGuid)
     // Resize vector to real size (some items can be skipped due to checks)
     packet.Items.resize(count);
 
+    if (!count)
+        packet.Reason = VENDOR_INVENTORY_REASON_INVENTORY_EMPTY;
+
     SendPacket(packet.Write());
 }