input: Write a function to convert all characters of a given string to lowercase. If the string is empty, return an empty string.
  def to_lower(string):
      return string.lower() if string else ""
  assert to_lower("HELLO")=="hello"
  assert to_lower("WORLD")=="world"
  assert to_lower("Hello, World!")=="hello, world!"
  assert to_lower("")==""
  assert to_lower("12345")=="12345"
output: def to_lower(string):
      return string.lower() if string else ""