input: Write a function to convert a given string to a different case. If the string contains both uppercase and lowercase letters, the function should swap them. If the string is empty, the function should return an empty string.
  assert convert_string_case("this Is A TeST")=="THIS iS a tEst"
  assert convert_string_case("ALLCAPS")=="allcaps"
  assert convert_string_case("nocaps")=="NOCAPS"
  assert convert_string_case("")==""
output: def convert_string_case(s):
      if not s:
          return ""
      result = []
      for char in s:
          if char.islower():
              result.append(char.upper())
          elif char.isupper():
              result.append(char.lower())
          else:
              result.append(char)
      return ''.join(result)