Fix #2020

[sete39]

What does this PR do?

Fixes #2020

This fix displays a warning if the c-TF-IDF is None when saving the model with save_ctfidf=True. This occurs when merging BERTopic models. Before this fix, this would cause a crash as discussed in the issue.

This is my first contribution so please do let me know if I did something wrong or if there any changes required!

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

Thanks for the PR! I left a small comment regarding the return statement.

[MaartenGr]

It all looks good to me, thank you for the work on this!