Update _topics.py

[ajdapretnar]

Proposed solution to not having enough topics and the call visualize_topics() resulting in an Error.

What does this PR do?

Fixes #2247.
Not sure whether this requires updating documentation or a test. If yes, happy to write it.

Before submitting

• This PR fixes a typo or improves the docs (if yes, ignore all other checks!).
• Did you read the contributor guideline?
• Was this discussed/approved via a Github issue? Please add a link to it if that's the case. Issue Warning when plotting MDS with two few topics #2247
• Did you make sure to update the documentation with your changes (if applicable)?
• Did you write any new necessary tests?

[MaartenGr]

Thank you for the PR! I left a couple of small questions/comments.

[MaartenGr]

Couple of questions:

• What makes you choose the value of < 5 here? Is it working for 4 topics but it is for 5?
• Should we raise an error rather than return nothing after a warning? By returning None, we might also have to change the typing.
• Could you extend the description a bit more? Perhaps explain that we need more topics in order to properly visualize them?