input: Write a function to convert a given string to its uppercase, lowercase, and title case representations. The function should return these cases as a tuple. If the given string is empty, it should return (None, None, None).
  assert convert_case("Hello World")==("HELLO WORLD", "hello world", "Hello World")
  assert convert_case("")==(None, None, None)
output: def convert_case(s):
      if s == "":
          return (None, None, None)
      return (s.upper(), s.lower(), s.title())