input: Write a function to generate a random string of a given length. The function should return a string consisting of lowercase letters. If the input length is invalid (less than or equal to 0), the function should return an empty string.
  
  assert random_string(6)=='abcdef' or random_string(6)=='ghijkl' or random_string(6)=='mnopqr' or random_string(6)=='stuvwx' or random_string(6)=='yzabcd'
  assert random_string(0)==''
  assert random_string(-3)==''
output: import random
  import string
  
  def random_string(length):
      if length <= 0:
          return ''
      return ''.join(random.choice(string.ascii_lowercase) for _ in range(length))