input: Write a function to generate prime numbers between 1 and a given number n. Return None. If the given number is less than 2, return None.
  assert generate_prime_numbers(10)
  assert generate_prime_numbers(20)
  assert generate_prime_numbers(2)
  assert generate_prime_numbers(1)==None
  assert generate_prime_numbers(0)==None
output: def generate_prime_numbers(n):
      if n < 2:
          return None
      primes = []
      for num in range(2, n + 1):
          for i in range(2, int(num**0.5) + 1):
              if num % i == 0:
                  break
          else:
              primes.append(num)
      return primes