Zero shot topic modelling

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### Desribe the bug

Getting a value error for undefined reason,
ValueError: Found array with 0 sample(s) (shape=(0, 1536)) while a minimum of 1 is required.

i have checked embeddings are working fine for test results.

```
import openai
from bertopic.representation import OpenAI
from bertopic import BERTopic
from langchain.embeddings import OpenAIEmbeddings

# Then use the following
my_key = "12323m2em2rm,2lr,2.f,."
client = openai.OpenAI(api_key = my_key)

embedding_model = OpenAIBackend(client, "text-embedding-ada-002")

summarization_prompt = """
I have a topic that is described by the following keywords: [KEYWORDS]
In this topic, the following documents are a small but representative subset of all documents in the topic:
[DOCUMENTS]

Based on the information above, please give a description of this topic in a one statement in the following format:
topic:
"""

# embedding_model = OpenAIBackend(client, "text-embedding-ada-002")

representation_model = OpenAI(client = client, model="gpt-4o", chat=True, prompt=summarization_prompt,
nr_docs=5, delay_in_seconds=3)

vectorizer_model = CountVectorizer(min_df=1)
topic_model = BERTopic(
embedding_model=embedding_model,
min_topic_size=25,
zeroshot_topic_list=zeroshot_topic_list,
zeroshot_min_similarity=0,
representation_model=representation_model
)

topics =topic_model.fit_transform(df['title'].values)
```

### Reproduction

```python
from bertopic import BERTopic

```

### BERTopic Version

0.16.3

No repository file or complete reproduction is provided. Start by tracing BERTopic.fit_transform through the zero-shot topic modelling path and reproduce the reported empty-array ValueError with the supplied configuration. Done means identifying the cause of the zero-sample array and documenting or correcting the behavior with a verified reproduction.

Written by the indexing model from the issue text.