input: Write a function to calculate the average of elements in a 2-dimensional array.
  def averageMatrix(matrix):
   sum = 0
   for row in matrix:
    for e in row:
     sum += e
  
   return sum / (len(matrix) * len(matrix[0]))
  assert averageMatrix([[3,2,6],[8,5,9],[4,7,1]]) == 5.0
  assert averageMatrix([[1,2,3],[4,5,6],[7,8,9]]) == 5.0
  assert averageMatrix([[1],[2],[3]]) == 2.0
  assert averageMatrix([[4,5],[6,7]]) == 5.5
  assert averageMatrix([[]]) == None
output: def averageMatrix(matrix):
      if not matrix or not matrix[0]:
          return None
      sum = 0
      count = 0
      for row in matrix:
          for e in row:
              sum += e
              count += 1
      return sum / count