input: Write a function to convert a given string to a different case by swapping the case of each character in the string. If the string is empty then return the 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("Python")=="pYTHON"
  assert convert_string_case("123abc")=="123ABC"
  assert convert_string_case("")==""
output: def convert_string_case(s):
      return s.swapcase()