[11490] Alow implicit cast ObjectGuid->uint64 and only explicit uint64->ObjectGuid

Now safe allow this casts in like way after completed convertion to ObjectGuid use.
Also simplify code in result allowed auto cast to uint64.

Please _not_ add new uint64 storages (local and in structures) for guid values.

src/game/QuestHandler.cpp

@@ -472,7 +472,7 @@ void WorldSession::HandlePushQuestToParty(WorldPacket& recvPacket)
                     continue;
                 }
 
-                if (!pPlayer->GetDividerGuid().IsEmpty())
+                if (pPlayer->GetDividerGuid())
                 {
                     _player->SendPushToPartyResponse(pPlayer, QUEST_PARTY_MSG_BUSY);
                     continue;