[XRay][test] Fix xray-empty-firstmbb.mir and delete incorrect xray-empty-function.mir

xray-empty-firstmbb.mir does not test the intended code path. Change
xray-instruction-threshold to 0 to exercise the code path.

Delete xray-empty-function.mir . Empty MachineFunction does not work.
Various passes (e.g. MachineDominatorTree) assume the presence of an
entry block.