input: Write a function to replace all occurrences of a character in a string with the provided replacement character. If the given string is empty then return the empty string.
  assert replace_all_occurrences("Hello World!", 'l', 'X')=="HeXXo WorXd!"
  assert replace_all_occurrences("Good Morning!", 'o', 'Y')=="GYyd MYrninyg!"
  assert replace_all_occurrences("Python", 'P', 'J')=="Jython"
  assert replace_all_occurrences("Language", '', 'Z')=="Language"
  assert replace_all_occurrences("", 'a', 'b')==""
output: def replace_all_occurrences(s, char, replacement):
      if not s:
          return s
      return s.replace(char, replacement)