input: Write a function that takes in an array of integers and returns the sum of the absolute values of those integers, multiplied by the product of their signs. If the array is empty, the function should return None.
  assert prod_signs([1, 2, 2, -4]) == -9
  assert prod_signs([0, 1]) == 0
  assert prod_signs([1, 1, 1, 2, 3, -1, 1]) == -10
  assert prod_signs([]) == None
  assert prod_signs([2, 4,1, 2, -1, -1, 9]) == 20
  assert prod_signs([-1, 1, -1, 1]) == 4
  assert prod_signs([-1, 1, 1, 1]) == -4
  assert prod_signs([-1, 1, 1, 0]) == 0
  
output: def prod_signs(arr):
      if not arr:
          return None
      product_of_signs = 1
      sum_of_abs = 0
      for num in arr:
          sum_of_abs += abs(num)
          if num != 0:
              product_of_signs *= (num // abs(num))
      return sum_of_abs * product_of_signs