input: Write a function to convert a given string to a different case by swapping the case of all characters in the string. If the input string is empty, return the original 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):
      return s.swapcase() if s else s