input: Write a function to convert all characters of a given string to lowercase. If the given string is empty, return an empty string.
  assert to_lower("HELLO")=="hello"
  assert to_lower("World")=="world"
  assert to_lower("aBcDeF")=="abcdef"
  assert to_lower("")==""
  assert to_lower("12345ABC")=="12345abc"
output: def to_lower(s):
      return s.lower()