`Chroma.Update_document()` Fails When `Document` Metadata Is Omitted

The `update_documents()` method in `langchain_chroma/vectorstores.py` forwards the document's `metadata` attribute directly to Chroma's `update()` method without normalizing empty dictionaries. Chroma's update API enforces that metadata must be a non-empty dict, so passing an empty dict (`{}`) triggers a validation error.

1. Create a `Chroma` instance with an embedding function. 2. Add a document with `Document(page_content='Original document')` and an ID using `add_documents()`. 3. Attempt to update the same document with `update_document(document_id='doc_1', document=Document(page_content='Updated document'))`. 4. Observe the `ValueError`.

This workaround introduces a semantic change: documents updated with empty metadata will retain their previous metadata rather than having it cleared. It also uses two separate update calls, which are not atomic; if the second call fails, the document text may be updated but the metadata not, creating an inconsistent state. Users should consider rolling back to a direct single update call once Chroma supports empty metadata dicts. Additionally, future Chroma versions might change the validation behavior, so pinning the Chroma version is advised until a native fix is available.