$before-1: map-merge((foo: 1, bar: 2), (baz: 3));
$before-2: map-merge((), (foo: 1, bar: 2));
$before-3: map-merge((foo: 1, bar: 2), ());
$before-4: map-merge((foo: 1, bar: 2), (foo: 3));

$after-1: (foo: 1, bar: 2, baz: 3);
$after-2: (foo: 1, bar: 2);
$after-3: (foo: 1, bar: 2);
$after-4: (foo: 3, bar: 2);

div {
  foo-1: $before-1 == $after-1;
  foo-2: $before-2 == $after-2;
  foo-3: $before-3 == $after-3;
  foo-4: $before-4 == $after-4;
}