Add additional case to remove entities without an identifier (db_id = '-')

nlm_gene.py

@@ -182,7 +182,7 @@ class NLMGeneDataset(datasets.GeneratorBasedBuilder):
             db_ids = db_ids.lstrip('-222,')
 
         # No listed entity for a mention
-        if db_ids in ['-1','-000','-111']:
+        if db_ids in ['-1','-000','-111','-']:
             normalized = []
 
         else: