input: Write a python function that, when given a list of strings, returns the one with the greatest length. If there are ties, return the first occurrence. For an empty list, return None.
  assert find_longest_string([]) == None
  assert find_longest_string(['a', 'b', 'c']) == 'a'
  assert find_longest_string(['a', 'bbb', 'cccc', 'ddd', 'eeee', 'xyz']) == 'cccc'
output: def find_longest_string(strings):
      if not strings:
          return None
      return max(strings, key=len)