input: Write a function to compute the mean and median of a given array. The function should take an array as input and return the mean and median. If the input array is empty, the function 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]) == (3.0, 3)
  assert mean_median([7, 3, 9, 1, 5]) == (5.0, 5)
  assert mean_median([]) == (None, None)
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