Reading an empty tagged x87 register doesnt raise exception nor substitutes QNaN Indefinite

Per the SDM, reading an x87 stack register whose tag is marked empty is a stack-underflow condition that sets IE and SF in the status word. When masked, the FPU substitutes the QNaN indefinite value into the register instead of its actual contents. FEX doesn't implement this at all, the same sequence leaves the value completely unchanged and never raises or substitutes anything.