find_topics(topic) crashes if topic is a 1-element list

Nobody has claimed this yet.

### Have you searched existing issues? 🔎

- [x] I have searched and found no existing issues

### Desribe the bug

`topic_model.find_topics(topic)` crashes if `topic` is a list with 1 string only.

It works if `topic` is a string or a list of strings with `len>1`

### Reproduction

```python
from bertopic import BERTopic
from sklearn.datasets import fetch_20newsgroups

docs = fetch_20newsgroups(subset='all', remove=('headers', 'footers', 'quotes'))['data']

topic_model = BERTopic()
topics, probs = topic_model.fit_transform(docs)
similar_topics, similarities = topic_model.find_topics(["internet"], top_n=50)
```

### BERTopic Version

0.17.4

Start by running the provided BERTopic reproduction with find_topics(["internet"], top_n=50), then inspect the find_topics entry point and its handling of single-item lists. Done means the one-element-list call completes consistently with the string and multi-item-list forms, with regression coverage for the reported case.

Written by the indexing model from the issue text.