Warning when using on empty maps

[pentzzsolt]

If at least one of the maps given to the function as an argument is empty, the function returns null and throws the warning:

recursive-map-merge() expects it's parameters to be map types!

This is because Sass sees empty maps as lists:

$empty-map: ();
if type-of($empty-map) == map {
	//This code is not executed.
}
if type-of($empty-map) == list {
	//This code is executed.
}

Because the function checks if both arguments are maps, it fails with empty maps.