Fix writing empty data to still write the data type
Fix writing empty data to still write the data type

* src/mat5.c (WriteData): Write the data type and 0 bytes even if data is
      NULL or there are 0 elements

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