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('')  # ''
import unittest

class MaxCharTestCase(unittest.TestCase):

    def test_with_many_chars(self):
        self.assertEqual(max_char('janbfthahdag'), 't')

    def test_with_a_single_char(self):
        self.assertEqual(max_char('j'), 'j')

    def test_with_empty_string(self):
        self.assertEqual(max_char(''), '')