input: Write a function to replace all occurrences of a character in a given string with the provided replacement character. If the string is empty, return the empty string.
  
  assert replace_all_occurrences("Hello World!", 'l', 'X')=="HeXXXo WorXd!"
  assert replace_all_occurrences("Python Programming", 'm', 'Z')=="Python PrograZZing"
  assert replace_all_occurrences("Example String", ' ', '-')=="Example-String"
  assert replace_all_occurrences("Repeated character", 'e', 'a')=="Rapated characta"
  assert replace_all_occurrences("", 'x', 'y')==""
output: def replace_all_occurrences(s, old, new):
      if not s:
          return ""
      return s.replace(old, new)