code: def count_blank_positions(self):
          
          blanks = 0
          for row_ndx in range(self.grid_height - 0):
              for col_ndx in range(self.grid_width - 0):
                  if self.get_tile(row_ndx, col_ndx) == EMPTY:
                      blanks += 1
          return blanks
docs: return a count of blank cells