"doc_length" doesn't work with llama3.1

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### Desribe the bug

I am using beropic with llama3.1 for topic modelling. My text is long, so I use doc_length in TextGeneration().

**Error:**

File "/home/bert/lib/python3.11/site-packages/bertopic/representation/_utils.py", line 57, in truncate_document
return truncated_document
^^^^^^^^^^^^^^^^^^
UnboundLocalError: cannot access local variable 'truncated_document' where it is not associated with a value

### Reproduction

```python
from bertopic import BERTopic

**here is my code:**

llama3 = TextGeneration(generator, prompt=prompt, nr_docs=4,doc_length=3000)
representation_model = {
"Llama3": llama3
}
topic_model = BERTopic(
embedding_model=embedding_model,
representation_model=representation_model,
umap_model=umap_model,
hdbscan_model=hdbscan_model,
nr_topics = nr_topics,
min_topic_size = 10,
verbose=True,
)

```

### BERTopic Version

0.16.4

Start in bertopic/representation/_utils.py at truncate_document, then reproduce the failure with TextGeneration(..., doc_length=3000) and the provided BERTopic setup. Done means long documents no longer raise UnboundLocalError and doc_length is applied as intended for llama3.1.

Written by the indexing model from the issue text.