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 (`""`) or `None` -- see sample tests.
from collections import Counter

def first_non_repeating_letter(string):
    count = Counter(s.lower() for s in string)
    
    for c in string:
        if count[c.lower()] == 1:
            return c
    
    return str()
from collections import Counter

def first_non_repeating_letter(string):
    count = Counter(s.lower() for s in string)
    
    for c in string:
        if count[c.lower()] == 1:
            return c
    
    return str()