Given a string containing unique letters, return a sorted string with the letters that don't appear in the string.
getMissing("abcdefgpqrstuvwxyz") ➞ "hijklmno"
getMissing("zyxwvutsrq") ➞ "abcdefghijklmnop"
getMissing("abc") ➞ "defghijklmnopqrstuvwxyz"
getMissing("abcdefghijklmnopqrstuvwxyz") ➞ ""