input: Write a function to convert a given string to a different case using the swapcase() method. If the string is empty, return the string as it is. The function should take a single string parameter and return the converted 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("Python3.9")=="pYTHON3.9"
  assert convert_string_case("12345")=="12345"
  assert convert_string_case("")==""
output: def convert_string_case(s):
      if s == "":
          return s
      return s.swapcase()