input: Write a function to read an array from the user and then determine the mean and median of the array. If the array is empty, return (None, None) as the mean and median cannot be calculated. 
  
  assert mean_median([10, 15, 20, 25, 30, 35]) == (22.5, 22.5)
  assert mean_median([10, 15, 20, 25, 30]) == (20.0, 20.0)
  assert mean_median([1, 2, 3, 4, 5, 6]) == (3.5, 3.5)
  assert mean_median([100, 200, 300, 400, 500, 600]) == (350.0, 350.0)
  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)