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()
  
  assert to_lower("HELLO") == "hello"
  assert to_lower("Python") == "python"
  assert to_lower("DATA SCIENCE") == "data science"
  assert to_lower("") == ""
output: def to_lower(string):
      return string.lower()