Return nullptr from LearnableAttacks when trying to retrieve from empty.

src/Library/CreatureData/LearnableAttacks.hpp

@@ -33,6 +33,9 @@ namespace CreatureLib::Library {
         const List<const AttackData*>& GetAttacksForLevel(uint8_t level) const;
 
         virtual const AttackData* GetRandomAttack(Arbutils::Random rand = Arbutils::Random()) const {
+            if (_distinctAttacks.empty()) {
+                return nullptr;
+            }
             auto val = rand.Get(_distinctAttacks.size());
             return *std::next(_distinctAttacks.begin(), val);
         }