input: Write a function to generate a random number between the given input numbers. If the input numbers are invalid or do not form a valid range, return None.
  assert random_num(0, 10) is not None
  assert random_num(0, 0) == 0
  assert random_num(1, 1) == 1
  assert random_num(10, 20) is not None
output: import random
  
  def random_num(a, b):
      if a > b:
          return None
      return random.randint(a, b)