input: Write a function to generate a random number between two given numbers (inclusive). The function should return the generated random number. If the input numbers are not valid, the function should return None.
  assert random_number_generator(2, 5) in [2, 3, 4, 5]
  assert random_number_generator(10, 20) in range(10, 21)
  assert random_number_generator(20, 10) == None
  assert random_number_generator(-5, -1) in [-5, -4, -3, -2, -1]
  assert random_number_generator(0, 0) == 0
output: import random
  
  def random_number_generator(a, b):
      if a > b:
          return None
      return random.randint(a, b)