input: Write a function to generate a random password using a given set of letters, numbers, and characters. The function should return the generated password as a string. If no input is provided, the function should use a predefined set of characters to generate the password.
  assert generate_random_password("")=="".join(random.sample("abcdefghijklmnopqrstuvwxyz01234567890ABCDEFGHIJKLMNOPQRSTUVWXYZ!@#$%^&*()?", 8))
  assert generate_random_password("abc123")=="".join(random.sample("abc123", 6))
  assert generate_random_password("ABCDEF")=="".join(random.sample("ABCDEF", 6))
  assert generate_random_password("123456")=="".join(random.sample("123456", 6))
  assert generate_random_password("")=="".join(random.sample("abcdefghijklmnopqrstuvwxyz01234567890ABCDEFGHIJKLMNOPQRSTUVWXYZ!@#$%^&*()?", 8))
output: import random
  
  def generate_random_password(characters=""):
      if not characters:
          characters = "abcdefghijklmnopqrstuvwxyz01234567890ABCDEFGHIJKLMNOPQRSTUVWXYZ!@#$%^&*()?"
      password_length = min(8, len(characters))
      return "".join(random.sample(characters, password_length))