input: Write a function to determine the mean and median of a given array. If the array is empty return (None, None). The function should also handle cases where the length of the array is odd or even.
  assert mean_median([10, 15, 20, 25, 30, 35])==(22.5, 22.5)
  assert mean_median([5, 10, 15, 20, 25])==(15.0, 15)
  assert mean_median([])==(None, None)
output: def mean_median(arr):
      if not arr:
          return None, None
      arr.sort()
      n = len(arr)
      mean = sum(arr) / n
      if n % 2 == 0:
          median = (arr[n//2 - 1] + arr[n//2]) / 2
      else:
          median = arr[n//2]
      return mean, median