refactor: IHierarchyObject Nullable IGameManager to Non-Nullable

Since in a typical use case of these classes they are already in the
hierarchy and it bloats the behaviour code with lots of null checks

Engine.Core/Abstract/Assignable/IHierarchyObject.cs

@@ -20,7 +20,7 @@ public interface IHierarchyObject : IEntity, INameable
     /// <summary>
     /// Gets the <see cref="IGameManager"/> associated with this <see cref="IEntity"/> , if any.
     /// </summary>
-    IGameManager? GameManager { get; }
+    IGameManager GameManager { get; }
 
     /// <summary>
     /// Indicates whether the <see cref="IEntity"/> is currently in the hierarchy.