input: Write a function to calculate the mean and median of a given array. The function should return both the mean and the median. If the array is empty, it should return None for both mean and median.
  assert mean_median([10, 15, 20, 25, 30, 35])==(22.5, 22.5)
  assert mean_median([1, 2, 3, 4, 5])==(3, 3)
  assert mean_median([])==(None, None)
  assert mean_median([9, 3, 44, 17, 15])==(17.6, 15)
  assert mean_median([5])==(5, 5)
output: def mean_median(arr):
      if not arr:
          return None, None
      arr.sort()
      mean = sum(arr) / len(arr)
      n = len(arr)
      if n % 2 == 0:
          median = (arr[n//2 - 1] + arr[n//2]) / 2
      else:
          median = arr[n//2]
      return mean, median