input: Write a function to calculate the mean and median of a given array. If the given array is empty then 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[floor(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([10, 15, 20, 25]) == (17.5, 17.5)
  assert mean_median([1, 2, 3, 4, 5]) == (3.0, 3)
  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