input: Write a python function to find the number in an array that is closest to zero.
  assert closest_to_zero([-2, 4, -5, -1]) == -1
  assert closest_to_zero([2, 3, -4, 1]) == 1
  assert closest_to_zero([7, 5, -3, 2]) == 2
output: def closest_to_zero(arr):
      if not arr:
          return None
      closest = arr[0]
      for num in arr:
          if abs(num) < abs(closest) or (abs(num) == abs(closest) and num > closest):
              closest = num
      return closest