Support generating representative docs when using online modelling

[27pchrisl]

The first time you fit an online model and request representative docs this works fine as every doc you fitted with will be in a cluster somewhere.

The second time you fit with a second set of docs, causing new clusters to be created, it's possible that none of the docs you passed fit into any of the existing clusters. This causes the _extract_representative_docs method to fail. Which results in you not being able to obtain representative docs for your new clusters.

This PR resolves this, so that _extract_representative_docs will return entries for all the topics, but some may be empty, based on the set of docs you provided.

[MaartenGr]

Thank you for the PR! You might have missed the CONTRIBUTING.md file where you can an overview of the steps we generally take when opening up a new PR. The reason why I link that file is that it is not clear to me what the exact issue is that you are fixing. Could you first create a reproducible example such that I can also see what the exact error/issue is? That way, it is easier to for me what the problem is and what could be potential ways to fix it.

[27pchrisl]

Hi @MaartenGr, I wrote this up in an issue.

Fixes #1620