When constructing a MTLIndirectAccelerationStructureInstanceDescriptor, one specifies an accelerationStructureID. In the equivalents in both Vulkan and DirectX12, one can set it to zero to be an "inactive" instance. However, on Metal, this field does not appear to have any documentation, and thus it is difficult to figure out if there is similar behavior (and no other Metal documentation seems to mention this). Does setting this field to 0 (i.e. null) disable the instance? If not, is there any other way to have an equivalent effect?