def empty(self, process_tile):
return ma.masked_values(np.zeros(process_tile.shape), <NUM_LIT:0>)<EOL>
Return empty data.

Parameters
----------
process_tile : ``BufferedTile``
    must be member of process ``TilePyramid``

Returns
-------
empty data : array or list
    empty array with correct data type for raster data or empty list
    for vector data