Add cell array with empty dataset elements and rename empty cell array variable
Add cell array with empty dataset elements and rename empty cell array variable

* test/test_mat.c (test_write_null): Change MATLAB variable name of
      empty cell array from cell_null_cells to cell_null, and add
      new cell array that is not empty but contains empty elements
      as the MATLAB variable cell_null_cells

tbeu/matio,MaartenBent/matio,CBL-ORION/matio,MaartenBent/matio,tbeu/matio,tbeu/matio,telehan/libmatio,martinichka/matio-coder,CBL-ORION/matio-sf-mirror,martinichka/matio-coder,tbeu/matio,CBL-ORION/matio,NJannasch/matio,MaartenBent/matio,MaartenBent/matio,martinichka/matio-coder,NJannasch/matio,CBL-ORION/matio-sf-mirror,telehan/libmatio