Core/Loot: Fixed crashes when generating loot on map that has no MapDifficulty data

Closes #29829

(cherry picked from commit 303dbd3393)

src/server/game/Entities/Item/ItemBonusMgr.cpp

@@ -61,6 +61,9 @@ void Load()
 
 ItemContext GetContextForPlayer(MapDifficultyEntry const* mapDifficulty, Player const* player)
 {
+    if (!mapDifficulty)
+        return ItemContext::NONE;
+
     auto evalContext = [](ItemContext currentContext, ItemContext newContext)
     {
         if (newContext == ItemContext::NONE)