Fix ner average grouping with no groups
Author: Narsil
Pull request: yes

# What does this PR do?
Fixes #https://github.com/huggingface/transformers/issues/24314

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Fixes # (issue)


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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- [ ] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
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Comments:
For bigger fixes I would add a test. This is small enough I think it's ok to skip. Let me know.

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