input: Write a function to generate a random string of the specified length containing alphanumeric characters. If the given length is less than or equal to zero, return an empty string.
  assert generate_random_string(8)
  assert generate_random_string(12)==False
  assert generate_random_string(0)==""
  assert generate_random_string(5)==""
  assert generate_random_string(16)
output: import random
  import string
  
  def generate_random_string(length):
      if length <= 0:
          return ""
      return ''.join(random.choices(string.ascii_letters + string.digits, k=length))