input: Write a python function to calculate the sum of elements in an array using recursion.
  assert arraySum([4, 8, 12, 16]) == 40
  assert arraySum([1, 2, 3, 4, 5]) == 15
  assert arraySum([]) == 0
  assert arraySum([10, -10, 10, -10]) == 0
output: def arraySum(arr):
      if not arr:
          return 0
      else:
          return arr[0] + arraySum(arr[1:])