Fix NPC obstacle handling (#13007)

Content.Server/NPC/NPCBlackboard.cs

@@ -42,6 +42,11 @@ public sealed class NPCBlackboard : IEnumerable<KeyValuePair<string, object>>
     /// </summary>
     public bool ReadOnly = false;
 
+    public void Clear()
+    {
+        _blackboard.Clear();
+    }
+
     public NPCBlackboard ShallowClone()
     {
         var dict = new NPCBlackboard();