input: Write a function to calculate the average of the elements in a 2-dimensional array. If the array is empty or contains empty subarrays, return 0.
  
  def averageMatrix(matrix):
   if not matrix or not matrix[0]:
    return 0
   sum = 0
   for row in matrix:
    for e in row:
     sum += e
   return sum / (len(matrix) * len(matrix[0]))
  
  a = [[3,2,6],
       [8,5,9],
       [4,7,1]]
  assert averageMatrix(a) == 5.0
  
  b = [[1, 2], [3, 4], [5, 6]]
  assert averageMatrix(b) == 3.5
  
  c = [[], [], []]
  assert averageMatrix(c) == 0
  
  d = []
  assert averageMatrix(d) == 0
output: def averageMatrix(matrix):
   if not matrix or not matrix[0]:
    return 0
   sum = 0
   for row in matrix:
    for e in row:
     sum += e
   return sum / (len(matrix) * len(matrix[0]))