[DXR] Clarify whether empty payloads and/or empty hit attribute structs are allowed

[jasilvanus]

Description

dxc explicitly forbids empty payload structs and empty hit attribute structs. This does not seem to be explicitly forbidden by the DXR spec.
Not sure what the intended behavior is, i.e. whether the spec should be updated, or whether this should be fixed in https://github.com/microsoft/DirectXShaderCompiler.

Empty payload example

struct [raypayload] EmptyPayload {};

[shader("miss")]
void MissEmptyPayload(inout EmptyPayload) {}

dxc -T lib_6_7 output:

empty_payload.hlsl:4:6: error: shader must include inout payload structure parameter.
void MissEmptyPayload(inout EmptyPayload) {}
     ^

Empty hit attribute struct example:

struct [raypayload] Payload { float value; };
struct Attributes {};

[shader("anyhit")]
void AnyHitEmptyAttributes(inout Payload, in Attributes) {}

dxc -T lib_6_7 output:

empty_payload.hlsl:5:6: error: shader must include attributes structure parameter.
void AnyHitEmptyAttributes(inout Payload, in Attributes) {}
     ^