input: Write a function to replace all occurrences of a character in a string with the provided replacement character. If the input string is empty then return the original string.
  assert replace_all_occurrences("Hello World!", 'l', 'X')== "HeXXo WorXd!"
  assert replace_all_occurrences("Programming is fun!", 'm', 'z')== "Prograzzing is fun!"
  assert replace_all_occurrences("No changes needed.", 'A', 'B')== "No changes needed."
  assert replace_all_occurrences("", 'a', 'b')== ""
output: def replace_all_occurrences(s, old, new):
      if not s:
          return s
      return s.replace(old, new)