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.

%!test <49379>
%! a1 = 1;
%! a2 = 2;
%! obj = class_bug49379 (a1, a2);
%! assert (obj.ctor_arg_names (), {"a1", "a2"})
%! meth (obj, a2, a1);
%! assert (obj.meth_arg_names (), {"obj", "a2", "a1"})
%! obj.meth (a2, a1);
%! assert (obj.meth_arg_names (), {"obj", "a2", "a1"})