input: Write a function to convert a given string to a different case using the swapcase() method. If the input string is empty, return an empty string.
  assert convert_string_case("this Is A TeST")=="THIS iS a tEst"
  assert convert_string_case("HeLlO WoRlD")=="hElLo wOrLd"
  assert convert_string_case("")==""
output: def convert_string_case(s):
      if s == "":
          return ""
      return s.swapcase()