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 for both mean and median.
  def mean_median(arr): 
        if not arr:
            return None, None
        arr_sum = 0 
        for i in range(len(arr)): 
              arr_sum += arr[i] 
        
        mean = arr_sum/len(arr) 
      
        arr = sorted(arr) 
        if len(arr) % 2 != 0: 
              median = arr[len(arr)//2] 
        else: 
              median = (arr[len(arr)//2] + arr[len(arr)//2 - 1]) / 2
        
        return mean, median
  
  assert mean_median([10, 15, 20, 25, 30, 35]) == (22.5, 22.5)
  assert mean_median([1, 2, 3, 4, 5, 6]) == (3.5, 3.5)
  assert mean_median([7, 3, 9, 5]) == (6.0, 5.0)
  assert mean_median([]) == (None, None)
output: def mean_median(arr): 
        if not arr:
            return None, None
        arr_sum = 0 
        for i in range(len(arr)): 
              arr_sum += arr[i] 
        
        mean = arr_sum/len(arr) 
      
        arr = sorted(arr) 
        if len(arr) % 2 != 0: 
              median = arr[len(arr)//2] 
        else: 
              median = (arr[len(arr)//2] + arr[len(arr)//2 - 1]) / 2
        
        return mean, median