input: Write a function to generate a random string of the specified length. If the given length is not provided or is zero, return an empty string.
  def random_string(length=6):
      if length <= 0:
          return ''
      letters = string.ascii_lowercase
      return ''.join(random.choice(letters) for i in range(length))
  
  assert random_string(6) != random_string(6)
  assert random_string(0) == ''
  assert len(random_string(6)) == 6
  assert len(random_string(0)) == 0
  assert len(random_string()) == 6
output: def random_string(length=6):
      if length <= 0:
          return ''
      letters = string.ascii_lowercase
      return ''.join(random.choice(letters) for i in range(length))