input: Write a function to convert all characters of a given string to lowercase. If the input 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("Python3") == "python3"
  assert to_lower("This Is A Test") == "this is a test"
  assert to_lower("") == ""
output: def to_lower(string):
      return string.lower() if string else ""