experimental[patch]: Fixes issue langchain-ai#17060 (langchain-ai#17062)

As described in issue langchain-ai#17060, in the case in which text has only one
sentence the following function fails. Checking for that and adding a
return case fixed the issue.

```python
    def split_text(self, text: str) -> List[str]:
        """Split text into multiple components."""
        # Splitting the essay on '.', '?', and '!'
        single_sentences_list = re.split(r"(?<=[.?!])\s+", text)
        sentences = [
            {"sentence": x, "index": i} for i, x in enumerate(single_sentences_list)
        ]
        sentences = combine_sentences(sentences)
        embeddings = self.embeddings.embed_documents(
            [x["combined_sentence"] for x in sentences]
        )
        for i, sentence in enumerate(sentences):
            sentence["combined_sentence_embedding"] = embeddings[i]
        distances, sentences = calculate_cosine_distances(sentences)
        start_index = 0

        # Create a list to hold the grouped sentences
        chunks = []
        breakpoint_percentile_threshold = 95
        breakpoint_distance_threshold = np.percentile(
            distances, breakpoint_percentile_threshold
        )  # If you want more chunks, lower the percentile cutoff

        indices_above_thresh = [
            i for i, x in enumerate(distances) if x > breakpoint_distance_threshold
        ]  # The indices of those breakpoints on your list

        # Iterate through the breakpoints to slice the sentences
        for index in indices_above_thresh:
            # The end index is the current breakpoint
            end_index = index

            # Slice the sentence_dicts from the current start index to the end index
            group = sentences[start_index : end_index + 1]
            combined_text = " ".join([d["sentence"] for d in group])
            chunks.append(combined_text)

            # Update the start index for the next group
            start_index = index + 1

        # The last group, if any sentences remain
        if start_index < len(sentences):
            combined_text = " ".join([d["sentence"] for d in sentences[start_index:]])
            chunks.append(combined_text)
        return chunks
```

Co-authored-by: Giulio Zani <[email protected]>

libs/experimental/langchain_experimental/text_splitter.py

@@ -85,6 +85,12 @@ def split_text(self, text: str) -> List[str]:
         """Split text into multiple components."""
         # Splitting the essay on '.', '?', and '!'
         single_sentences_list = re.split(r"(?<=[.?!])\s+", text)
+
+        # having len(single_sentences_list) == 1 would cause the following
+        # np.percentile to fail.
+        if len(single_sentences_list) == 1:
+            return single_sentences_list
+
         sentences = [
             {"sentence": x, "index": i} for i, x in enumerate(single_sentences_list)
         ]