input: Write a function to generate a random string of a specified length. If the input length is less than or equal to zero, return an empty string. Otherwise, return a string consisting of lowercase letters of the specified length.
  
  def generate_random_string(length):
      if length <= 0:
          return ""
      letters = string.ascii_lowercase
      return ''.join(random.choice(letters) for i in range(length))
  
  assert generate_random_string(6) != generate_random_string(6)
  assert generate_random_string(0) == ""
  assert generate_random_string(-5) == ""
  assert len(generate_random_string(10)) == 10
  assert all(c in string.ascii_lowercase for c in generate_random_string(20))
output: def generate_random_string(length):
      if length <= 0:
          return ""
      letters = string.ascii_lowercase
      return ''.join(random.choice(letters) for i in range(length))