If the input mesh had no materials already, the new material would
become the only material on the mesh, meaning the material was
added to all of the faces, instead of just the selected faces.
The mesh primitive nodes in geometry nodes already add an empty
slot by default, so this only affects outside geometry.
The fix is just adding an empty slot before the new slot, so the
non-selected material indices can still point to an empty slot.
Differential Revision: https://developer.blender.org/D13654
8f89196be2