input: Write a function to generate a random string of a given length. The string should only contain lowercase letters. If the input length is zero, return an empty string.
  assert random_string(6)=='abcxyz'
  assert random_string(0)==''
  assert random_string(8)=='aeghfzmq'
  assert random_string(3)=='nrt'
  assert random_string(4)=='skfe'
output: import random
  import string
  
  def random_string(length):
      if length == 0:
          return ''
      return ''.join(random.choices(string.ascii_lowercase, k=length))