input: Write a python function to find the longest string in a list of strings.
  assert find_longest_string(['hello', 'world', 'this', 'is', 'awesome']) == 'awesome'
  assert find_longest_string(['one', 'three', 'seventeen', 'four']) == 'seventeen'
  assert find_longest_string(['short', 'longer', 'longest']) == 'longest'
output: def find_longest_string(strings):
      if not strings:
          return None
      return max(strings, key=len)