`Chroma.update_document()` fails when `Document` metadata is omitted

### Submission checklist - [x] This is a bug, not a usage question. - [x] I added a clear and descriptive title that summarizes this issue. - [x] I used the GitHub search to find a similar question and didn't find it. - [x] I am sure that this is a bug in LangChain rather than my code. - [x] The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package). - [x] This is not related to the langchain-community package. - [x] I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS. ### Package (Required) - [ ] langchain - [ ] langchain-openai - [ ] langchain-anthropic - [ ] langchain-classic - [ ] langchain-core - [ ] langchain-model-profiles - [ ] langchain-tests - [ ] langchain-text-splitters - [x] langchain-chroma - [ ] langchain-deepseek - [ ] langchain-exa - [ ] langchain-fireworks - [ ] langchain-groq - [ ] langchain-huggingface - [ ] langchain-mistralai - [ ] langchain-nomic - [ ] langchain-ollama - [ ] langchain-openrouter - [ ] langchain-perplexity - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Related Issues / PRs _No response_ ### Reproduction Steps / Example Code (Python) ```python import os import getpass from dotenv import load_dotenv from langchain_openai import OpenAIEmbeddings from langchain_core.documents import Document from langchain_chroma import Chroma # Step 1: Load OpenAI API Key load_dotenv(override=True) if "OPENAI_API_KEY" not in os.environ: os.environ["OPENAI_API_KEY"] = getpass.getpass("OpenAI API Key: ") # Step 2: Create Embedder embedder = OpenAIEmbeddings(model="text-embedding-3-large") # Step 3: Create Vector Store vector_store = Chroma(collection_name="test_collection", embedding_function=embedder) # Step 4: Add Document vector_store.add_documents( documents=[Document(page_content="Original document")], ids=["doc_1"] ) # Step 5: Update Document vector_store.update_document( document_id="doc_1", doc…