Fix empty structures in HDF Schema 1
Fix empty structures in HDF Schema 1

* src/mat73.c (Mat_VarReadNextInfo73): If object type is H5G_DATASET,
      also check if the class type is a structure to handle empty
      structures written as a dataset

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