added health_fact dataset 
Hi @lhoestq,
Initially I tried int(-1) only in place of nan labels and missing values but I kept on getting this error ```pyarrow.lib.ArrowTypeError: Expected bytes, got a 'int' object``` maybe because I'm sending int values (-1) to objects which are string type
Added dataset Explainable Fact-Checking for Public Health Claims (dataset_id: health_fact)