Update topic mapping when all documents match zero-shot topics

[huisman]

Fix topic labels when all documents match zero-shot topics by updating the topic id mapping.

What does this PR do?

When all documents match zero-shot topics, the labels for the topics are not set to the zero-shot topic name.
This PR tries to fix this by updating the topic id/mapping and updating the topic sizes when there are no non-zero-shot topics.

Fixes #2447

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• This PR fixes a typo or improves the docs (if yes, ignore all other checks!).
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[MaartenGr]

Thank you for the PR! Could you perhaps give an example of how it fixes the issue? That way, I can run some tests.

Also, did you test whether the predictions are still correct? I wonder whether the update topics would properly propagate the predictions through the mapping.

Lastly, I think you duplicated this:

# All documents matches zero-shot topics
documents = assigned_documents
embeddings = assigned_embeddings

Which might break things depending on how it was ordered.