vkd3d-shader/hlsl: Don't produce a parse error on empty buffer_body.

Approved-by: Giovanni Mascellani (@giomasce)
Approved-by: Henri Verbeet (@hverbeet)
Approved-by: Alexandre Julliard (@julliard)
Merge-Request: https://gitlab.winehq.org/wine/vkd3d/-/merge_requests/48

tests/cbuffer.shader_test

@@ -0,0 +1,17 @@
+[pixel shader]
+// Test empty constant buffer.
+cbuffer Constants : register(b1)
+{
+};
+
+float4 foo;
+
+float4 main() : sv_target
+{
+    return foo;
+}
+
+[test]
+uniform 0 float4 1.0 2.0 3.0 4.0
+draw quad
+probe all rgba (1.0, 2.0, 3.0, 4.0)