[AI] Fill Collision Underneath for Navmesh does not work for ISM instances

Although the ISM component allows for enabling the flag for Fill Collision Underneath for Navmesh, it does not add the modifier for this to the navigation data for its instances. 

1. Create an Actor BP
2. Add InstancedStaticMesh component to BP
3. Set the mesh of the ISM component to use SM_Cube
4. Enable the flag for Fill Collision Underneath for Navmesh under the navigation section of the ISM component
5. In the Event Graph, add an Add Instance node off the Begin Play event. Set the transform to use the actor's location and set the scale to (5.0, 5.0, 5.0)
6. Add the BP to a level with dynamic navmesh setup
7. Set the navmesh bounds volume Z-scale to 10 to cover the entire cube
   1. The cube should have navmesh on top of it
8. SIE
9. Maneuver the camera into the mesh

Expected Result: The navmesh is removed from the inside of the cube.

Actual Result: Navmesh is still created inside the cube.