$("li.par_cat").click(function() {
	var el_count = $(this).nextUntil("li.par_cat").size();
	if(el_count == 0){
		// I'll add something here later.
	}else{
	$(this).nextUntil("li.par_cat").each(function() {
		$(this).slideToggle("slow");
	});
	return false;
	}
});

Catches click, if element has nextUntil elements, triggers a slideDown of child elements and returns false (to stop the anchor), if there are no nextUntil elements, does nothing and allows the click through (the anchor works).

jQuery  find li elements until, if has nextUntil elements

jquery

jQuery