input: Write a function to replace all occurrences of a given character in a string with the provided replacement character. If the input string is empty, return an empty string.
  assert replace_all_occurrences("Hello World!", 'l', 'X')=="HeXXo WorXd!"
  assert replace_all_occurrences("Programming is fun!", 'm', 'Y')=="PrograYing is fun!"
  assert replace_all_occurrences("Python", 'o', 'a')=="Pythan"
  assert replace_all_occurrences("", 'a', 'b')==""
  assert replace_all_occurrences("Hello World!", 'z', 'X')=="Hello World!"
output: def replace_all_occurrences(s, char, replacement):
      if not s:
          return ""
      return s.replace(char, replacement)