input: Write a function to convert a given string to its opposite case (i.e., lowercase to uppercase and vice versa). If the input string is empty, return an empty string.
  def convert_string_case(string):
      if not string:
          return ""
      return string.swapcase()
  
  assert convert_string_case("this Is A TeST")=="THIS iS a tEst"
  assert convert_string_case("HELLO world")=="hello WORLD"
  assert convert_string_case("")==""
  assert convert_string_case("12345")=="12345"
  assert convert_string_case("Python3")=="pYTHON3"
output: def convert_string_case(string):
      if not string:
          return ""
      return string.swapcase()