input: Write a function to 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(['bar', 'baz', 'quux']) == 'quux'
  assert longest_string([]) == ''
  assert longest_string(['a', 'bb', 'ccc', 'dddd']) == 'dddd'
  assert longest_string(['short', 'longer', 'shortest']) == 'longer'
output: def longest_string(strings):
      if not strings:
          return ''
      return max(strings, key=len)