Fix ner average grouping with no groups (#24319)

Fixes #https://github.com/huggingface/transformers/issues/24314

src/transformers/pipelines/token_classification.py

@@ -491,6 +491,7 @@ class TokenClassificationPipeline(ChunkPipeline):
                 word_entities.append(self.aggregate_word(word_group, aggregation_strategy))
                 word_group = [entity]
         # Last item
+        if word_group is not None:
             word_entities.append(self.aggregate_word(word_group, aggregation_strategy))
         return word_entities