transformers is broken due to pr-406
**Describe the bug**
Not every model with `cls_token` has the `extra_output` in the outputs.

**Describe how you solve it**
add a checkpoint to check if a valid `extra_output` is returned.

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**Environment**
<!-- Run `jina --version-full` and copy paste the output here -->

**Screenshots**
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diff --git a/jina/executors/encoders/nlp/transformer.py b/jina/executors/encoders/nlp/transformer.py
index 0305a74c397..33367f0162b 100644
--- a/jina/executors/encoders/nlp/transformer.py
+++ b/jina/executors/encoders/nlp/transformer.py
@@ -65,7 +65,7 @@ class BaseTransformerEncoder(BaseEncoder):
             _mask_ids_batch = self.tensor2array(mask_ids_batch)
             _seq_output = self.tensor2array(seq_output)
             if self.pooling_strategy == 'cls':
-                if hasattr(self._tokenizer, 'cls_token'):
+                if hasattr(self._tokenizer, 'cls_token') and len(extra_output) > 0:
                     output = self.tensor2array(extra_output[0])
                 else:
                     output = reduce_cls(_seq_output, _mask_ids_batch, self.cls_pos)
['jina/executors/encoders/nlp/transformer.py']