input: Write a function to generate a random string of a specified length using lowercase letters from the English alphabet. If the length is not provided or is zero, return an empty string.
  import random
  import string
  
  def random_string(length):
      if length < 1:
          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(-1)) == 0
  assert all(c in string.ascii_lowercase for c in random_string(10))
output: def random_string(length):
      if length < 1:
          return ""
      letters = string.ascii_lowercase
      return ''.join(random.choice(letters) for i in range(length))