Max character

Write a function max_char that receives a string and returns the "maximum" character in it. Max is defined as a < b < ... < z ignoring uppercase chars (all strings provided will be lowercase). Important: If the string received is empty, the result should be an empty string. Example:

max_char('azg')  # 'z'
max_char('')  # ''

Test Cases

test with empty string - Run Test

def test_with_empty_string():
    assert max_char('') == ''

test with a single char - Run Test

def test_with_a_single_char():
    assert max_char('j') == 'j'

test with many chars - Run Test

def test_with_many_chars():
    assert max_char('janbfthahdag') == 't'