// store the links parent selectors as variables
var $next = $(".next");
var $prev = $(".prev"); 
// if the prev posts link doesn't exist but the next link does
if ($prev.children().length == 0 && $next.children().length > 0) {
  // add a span with prev
  $prev.append('<span class="disabled">Prev</span>');
// if the next posts link doesn't exist but the prev one does
} else if ($next.children().length == 0 && $prev.children().length > 0) {
  // add a span with next
  $next.append('<span class="disabled">Next</span>');
};