Assassin's Creed: Valhalla sets a null index buffer, but Windows allows null for root descriptors too.
-- v2: tests: Test index buffer location zero in test_draw_indexed_instanced(). vkd3d: Check for index buffer location zero. tests: Test null addresses in test_update_root_descriptors(). vkd3d: Support null address for SRV/UAV root descriptors.