input: Write a function to generate a random string of a given length using lowercase letters. If the length is not provided, the function should return a string of length 6 by default. The function should also handle cases where the length is less than or equal to zero by returning an empty string.
  assert random_string(6)=='random' # Output will vary
  assert random_string()=='default' # Output will vary
  assert random_string(0)=='expected output'==''
  assert random_string(-5)=='expected output'==''
output: import random
  import string
  
  def random_string(length=6):
      if length <= 0:
          return ''
      return ''.join(random.choices(string.ascii_lowercase, k=length))