input: Write a function to convert a given string to a different case by swapping each character's case. If the character is uppercase, convert it to lowercase and vice versa. If the string is empty, 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("12345")=="12345"
  assert convert_string_case("")==""
output: def convert_string_case(s):
      return s.swapcase()