fix 'best_non_null_entry' is None error

examples/run_squad.py

@@ -573,6 +573,13 @@ def write_predictions(all_examples, all_features, all_results, n_best_size,
                         text="",
                         start_logit=null_start_logit,
                         end_logit=null_end_logit))
+                
+            # In very rare edge cases we could only have single null prediction.
+            # So we just create a nonce prediction in this case to avoid failure.
+            if len(nbest)==1:
+                nbest.insert(0,
+                    _NbestPrediction(text="empty", start_logit=0.0, end_logit=0.0))
+                
         # In very rare edge cases we could have no valid predictions. So we
         # just create a nonce prediction in this case to avoid failure.
         if not nbest: