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.

function r = bug_60237 ()
  d = 2;
  function c = bm (a)
    c = a + d;
  endfunction;
  r = ancall (@(a) bm (a), 2);
endfunction

function r = ancall (f, a)
  r = f (a) + 1;
endfunction