Write a function named first_non_repeating_letter† that takes a string input, and returns the first character that is not
repeated anywhere in the string.
For example, if given the input 'stress', the function should return 't', since the letter t only occurs once in the string, and
occurs first in the string.
As an added challenge, upper- and lowercase letters are considered the same character, but the function should return the correct case
for the initial letter. For example, the input 'sTreSS' should return 'T'.
If a string contains all repeating characters, it should return an empty string ("");
† Note: the function is called firstNonRepeatingLetter for historical reasons, but your function should handle any Unicode
character.