input: Write a function to replace all occurrences of a character in a given string with the provided replacement character. If the given string is empty, return the empty string. The function should return the modified string.
  assert replace_all_occurrences("Hello World!", 'l', 'X')=="HeXXo WorXd!"
  assert replace_all_occurrences("Hello World!", 'o', 'Y')=="HellY WYrld!"
  assert replace_all_occurrences("Hello World!", ' ', '-')=="Hello-World!"
  assert replace_all_occurrences("", 'a', 'b')==""
output: def replace_all_occurrences(s, char, replacement):
      if not s:
          return s
      return s.replace(char, replacement)