input: Write a function to convert a given string to a different case. The function should swap the case of each character in the string. If the given string is empty, it should return an empty string.
  assert convert_string_case("Hello World")=="hELLO wORLD"
  assert convert_string_case("Python3")=="pYTHON3"
  assert convert_string_case("123abc")=="123ABC"
  assert convert_string_case("")==""
output: def convert_string_case(s):
      return s.swapcase()