text: Title: c_tf_idf_.indptr is None when attempting to save merged model
  Body: After using `BERTopic.merge_models`, I am unable to save the resulting model. Here is some repro code:
  ```python
  import bertopic
  from bertopic import BERTopic
  from sklearn.datasets import fetch_20newsgroups
  from sklearn.decomposition import PCA
  
  print(f'Using bertopic version {bertopic.__version__}')
  
  embedding_model = 'all-mpnet-base-v2'
  
  def new_topic_model():
      return BERTopic(
          embedding_model=embedding_model,
          # Using PCA because UMAP segfaults on my machine
          umap_model=PCA(n_components=10, random_state=40),
      )
  
  docs = fetch_20newsgroups(subset='all',  remove=('headers', 'footers', 'quotes'))['data']
  docs_a = docs[:400]
  docs_b = docs[400:500]
  
  topic_model_a = new_topic_model()
  topic_model_a.fit(docs_a)
  # No problem saving topic_model_a
  topic_model_a.save(
      './topic_model_a',
      serialization='safetensors',
      save_embedding_model=embedding_model,
      save_ctfidf=True,
  )
  
  topic_model_b = new_topic_model()
  topic_model_b.fit(docs_b)
  # No problem saving topic_model_b
  topic_model_b.save(
      './topic_model_b',
      serialization='safetensors',
      save_embedding_model=embedding_model,
      save_ctfidf=True,
  )
  
  topic_model = BERTopic.merge_models([topic_model_a, topic_model_b])
  # Problem here: AttributeError: 'NoneType' object has no attribute 'indptr'
  topic_model.save(
      './topic_model',
      serialization='safetensors',
      save_embedding_model=embedding_model,
      save_ctfidf=True,
  )
  ```
  
  I get the following output:
  ```
  % python3 repro.py
  Using bertopic version 0.16.2
  Traceback (most recent call last):
    File "[...]/BERTopic/repro.py", line 43, in <module>
      topic_model.save(
    File "[...]/BERTopic/bertopic/_bertopic.py", line 3112, in save
      save_utils.save_ctfidf(model=self, save_directory=save_directory, serialization=serialization)
    File "[...]/BERTopic/bertopic/_save_utils.py", line 318, in save_ctfidf
      indptr = torch.from_numpy(model.c_tf_idf_.indptr)
                                ^^^^^^^^^^^^^^^^^^^^^^
  AttributeError: 'NoneType' object has no attribute 'indptr'
  ```