input: Write a function to generate a random string of a given length using lowercase letters. The function should be able to handle any non-negative integer input for the length of the string. If the input length is zero, return an empty string.
  assert random_string(6) # This should return a random string of length 6
  assert random_string(0)==''
  assert len(random_string(10))==10
output: import random
  import string
  
  def random_string(length):
      if length == 0:
          return ''
      return ''.join(random.choices(string.ascii_lowercase, k=length))