input: Write a function to replace all occurrences of a character in a string with the provided replacement character. If the string is empty, return the empty string.
  assert replace_all_occurrences("Hello World!", 'l', 'X') == "HeXXo WorXd!"
  assert replace_all_occurrences("Python Programming", 'P', 'Y') == "ython Yrogramming"
  assert replace_all_occurrences("Python", 'z', 'a') == "Python"
  assert replace_all_occurrences("", 'a', 'b') == ""
output: def replace_all_occurrences(s, old, new):
      if not s:
          return s
      return s.replace(old, new)