input: Write a function to find and return the longest string in a given list of strings. If the list is empty, return an empty string.
  assert longest_string(['foo', 'test', 'longstring', 'bar']) == 'longstring'
  assert longest_string(['short', 'longerstring', 'medium']) == 'longerstring'
  assert longest_string(['one', 'two', 'three', 'four']) == 'three'
  assert longest_string([]) == ''
output: def longest_string(strings):
      if not strings:
          return ''
      return max(strings, key=len)