input: Write a function to generate a random string of a given length. The string should consist of lowercase letters. If no length is provided, the default length should be 6. The function should return the generated string.
  assert random_string(6) in [''.join(random.choice(string.ascii_lowercase) for i in range(6)) for _ in range(1000)]
  assert len(random_string(4)) == 4
  assert len(random_string()) == 6
  assert random_string(0) == ''
output: import random
  import string
  
  def random_string(length=6):
      return ''.join(random.choice(string.ascii_lowercase) for _ in range(length))