input: Write a function to generate a random password with a minimum length of 8 characters and it should contain at least one number and one special character. If the length constraint is not met, return None.
  assert generate_password() is not None
  assert len(generate_password()) == 8
  assert any(char.isdigit() for char in generate_password())
  assert any(char in string.punctuation for char in generate_password())
  assert not generate_password(length=7)
output: import random
  import string
  
  def generate_password(length=8):
      if length < 8:
          return None
      chars = string.ascii_letters + string.digits + string.punctuation
      while True:
          password = ''.join(random.choice(chars) for _ in range(length))
          if any(char.isdigit() for char in password) and any(char in string.punctuation for char in password):
              return password