input: Write a python function to find the frequency of each character in a given string.
  assert char_frequency("Hello World") == {'H': 1, 'e': 1, 'l': 3, 'o': 2, ' ': 1, 'W': 1, 'r': 1, 'd': 1}
  assert char_frequency("test string") == {'t': 3, 'e': 1, 's': 2, ' ': 1, 'r': 1, 'i': 1, 'n': 1, 'g': 1}
  assert char_frequency("") == {}
output: def char_frequency(s):
      freq = {}
      for char in s:
          if char in freq:
              freq[char] += 1
          else:
              freq[char] = 1
      return freq