input: Write a function to convert a given string to a different case, such as converting all characters to uppercase or lowercase. The function should take a string as input and return the string with its case swapped.
  assert convert_string_case("this Is A TeST")=="THIS iS a tEst"
  assert convert_string_case("HELLO")=="hello"
  assert convert_string_case("world")=="WORLD"
  assert convert_string_case("")==""
  assert convert_string_case("123")=="123"
output: def convert_string_case(s):
      return ''.join([char.lower() if char.isupper() else char.upper() for char in s])