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, return an empty string.
  assert replace_all_occurrences("Hello World!", 'l', 'X')=="HeXXo WorXd!"
  assert replace_all_occurrences("Python Programming", 'm', 'Z')=="Python PrograZZing"
  assert replace_all_occurrences("One Two Three", 'e', 'A')=="OnA Two ThrAe"
  assert replace_all_occurrences("", 'a', 'b')==""
  assert replace_all_occurrences("No replacements", 'X', 'Y')=="No replacements"
output: def replace_all_occurrences(s, char, replacement):
      if not s:
          return ""
      return s.replace(char, replacement)