HLSL: Allow empty struct initializers

Review request: does this harm GLSL in any way?

Test/hlsl.emptystruct.init.vert

@@ -0,0 +1,8 @@
+struct Test { };
+
+static const Test Test_Empty;
+
+float4 main(in uint vertexIndex : VERTEXID) : VS_OUT_POSITION
+{
+    return 0;
+}