Some rows contain ''None" labels.

[dongheechoi]

When I run bash scripts/build_biorex_datasets.sh, the error occurs.

https://github.com/ncbi/BioREx/blob/0031c52d2dd0d7fb8e3b84e256d8fbf73c4bd463/src/run_ncbi_rel_exp.py#L112 Here, the code emits an error, because it cannot find a label when the label is ''.

I think the https://github.com/ncbi/BioREx/blob/0031c52d2dd0d7fb8e3b84e256d8fbf73c4bd463/src/run_ncbi_rel_exp.py#L148 should be changed. The None label is changed into "'' for this code.