input: Write a function to generate a random string of a specified length. The string should consist of lowercase English letters. If an invalid length is provided (i.e., a non-positive integer), the function should return an empty string. The function should be named `random_string` and take a single integer parameter `length`.
  assert random_string(6) != random_string(6)
  assert random_string(-1) == ""
  assert len(random_string(3)) == 3
  assert all(c in string.ascii_lowercase for c in random_string(5))
output: import random
  import string
  
  def random_string(length):
      if length <= 0:
          return ""
      return ''.join(random.choice(string.ascii_lowercase) for _ in range(length))