sparse-xpow.cc: Improve consistency between sparse and full matrices (bug #63080) sparse-xpow.cc: Return 0x0 matrix for empty inputs of all sizes. Return early for all-zeros matrix with positive power. Add FIXME for all-zeros matrix with negative power. Add BISTs.

## u_vr.m

## define and exectute "__demo__" once
eval ("function __demo__ ();  df_vr;  v = vr * 2; endfunction");
__demo__;

## clear definition of "__demo__"
clear __demo__

## define and exectute "__demo__" once more
eval ("function __demo__ ();  df_vr;  v = vr * 2; endfunction");
__demo__;