input: Write a Python function that accepts an array of integers and calculates the sum of their absolute values, then multiplies that sum 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
      sum_abs = sum(abs(x) for x in arr)
      prod_sign = 1
      for num in arr:
          if num == 0:
              return 0
          elif num < 0:
              prod_sign *= -1
      return sum_abs * prod_sign