Pydantic validation error when using `retriever.invoke()`

### Checked other resources

- [x] I added a very descriptive title to 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] I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS.

### Example Code

vector_store = Chroma(persist_directory = 'misc/conversations', embedding_function = embeddings)
retriever = vector_store.as_retriever(search_kwargs={"k": 1})
retrieved_docs = retriever.invoke(query)

### Error Message and Stack Trace (if applicable)

Exception occurred
Traceback (most recent call last):
  File "/home/userA/test_dir/app.py", line 137, in analyze_query
    response_list = await agent.ainvoke({"messages": message, "session_id": session_id}, config = {"configurable": {"thread_id": session_id}})
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/userA/test_dir/venv/lib/python3.11/site-packages/langgraph/pregel/init.py", line 2850, in ainvoke
    async for chunk in self.astream(
  File "/home/userA/test_dir/venv/lib/python3.11/site-packages/langgraph/pregel/init.py", line 2732, in astream
    async for _ in runner.atick(
  File "/home/userA/test_dir/graph.py", line 154, in validate_conversations
    retrieved_docs = retriever.invoke(query)
                     ^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/userA/test_dir/venv/lib/python3.11/site-packages/langchain_core/retrievers.py", line 258, in invoke
    result = self._get_relevant_documents(
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/userA/test_dir/venv/lib/python3.11/site-packages/langchain_core/vectorstores/base.py", line 1079, in _get_relevant_documents
    docs = self.vectorstore.similarity_search(query, **_kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/userA/test_dir/venv/lib/python3.11/site-packages/langchain_chroma/vectorstores.py", line 608, in similarity_search
    docs_and_scores = self.similarity_search_with_score(
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/userA/test_dir/venv/lib/python3.11/site-packages/langchain_chroma/vectorstores.py", line 714, in similarity_search_with_score
    return _results_to_docs_and_scores(results)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/userA/test_dir/venv/lib/python3.11/site-packages/langchain_chroma/vectorstores.py", line 45, in _results_to_docs_and_scores
    return [
           ^
  File "/home/userA/test_dir/venv/lib/python3.11/site-packages/langchain_chroma/vectorstores.py", line 49, in <listcomp>
    Document(page_content=result[0], metadata=result[1] or {}, id=result[2]),
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/userA/test_dir/venv/lib/python3.11/site-packages/langchain_core/documents/base.py", line 289, in init
    super().init(page_content=page_content, **kwargs)  # type: ignore[call-arg]
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/userA/test_dir/venv/lib/python3.11/site-packages/langchain_core/load/serializable.py", line 130, in init
    super().init(*args, **kwargs)
  File "/home/userA/test_dir/venv/lib/python3.11/site-packages/pydantic/main.py", line 253, in init
    validated_self = self.pydantic_validator.validate_python(data, self_instance=self)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pydantic_core._pydantic_core.ValidationError: 1 validation error for Document
page_content
  Input should be a valid string [type=string_type, input_value=None, input_type=NoneType]
    For further information visit https://errors.pydantic.dev/2.11/v/string_type 
During task with name 'Conversations (Validate)' and id '0bfe5df4-b17a-8700-dcea-f806305057ff'

### Description

The error log basically mentions that **the document retrieved from vector DB had page_content = None**. I checked the contents of the vector DB; none of the documents had null page_content; further, it is not even possible to save a document with page_content = None. So, this appears to be a bug.

### System Info

System Information
------------------
> OS:  Windows
> OS Version:  10.0.26100
> Python Version:  3.13.0 (tags/v3.13.0:60403a5, Oct  7 2024, 09:38:07) [MSC v.1941 64 bit (AMD64)]

Package Information
-------------------
> langchain_core: 0.3.56
> langchain: 0.3.24
> langchain_community: 0.3.21
> langsmith: 0.3.27
> langchain_chroma: 0.2.3
> langchain_cohere: 0.4.3
> langchain_groq: 0.3.2
> langchain_milvus: 0.1.10
> langchain_openai: 0.3.14
> langchain_qdrant: 0.2.0
> langchain_text_splitters: 0.3.8
> langgraph_sdk: 0.1.61

Optional packages not installed
-------------------------------
> langserve

Other Dependencies
------------------
> aiohttp<4.0.0,>=3.8.3: Installed. No version info available.
> async-timeout<5.0.0,>=4.0.0;: Installed. No version info available.
> chromadb!=0.5.10,!=0.5.11,!=0.5.12,!=0.5.4,!=0.5.5,!=0.5.7,!=0.5.9,<0.7.0,>=0.4.0: Installed. No version info available.
> cohere: 5.14.2
> dataclasses-json<0.7,>=0.5.7: Installed. No version info available.
> fastembed: Installed. No version info available.
> groq<1,>=0.4.1: Installed. No version info available.
> httpx: 0.28.1
> httpx-sse<1.0.0,>=0.4.0: Installed. No version info available.
> jsonpatch<2.0,>=1.33: Installed. No version info available.
> langchain-anthropic;: Installed. No version info available.
> langchain-aws;: Installed. No version info available.
> langchain-azure-ai;: Installed. No version info available.
> langchain-cohere;: Installed. No version info available.
> langchain-community;: Installed. No version info available.
> langchain-core<1.0.0,>=0.3.49: Installed. No version info available.
> langchain-core<1.0.0,>=0.3.51: Installed. No version info available.
> langchain-core<1.0.0,>=0.3.53: Installed. No version info available.
> langchain-core<1.0.0,>=0.3.55: Installed. No version info available.
> langchain-core>=0.3.52: Installed. No version info available.
> langchain-deepseek;: Installed. No version info available.
> langchain-fireworks;: Installed. No version info available.
> langchain-google-genai;: Installed. No version info available.
> langchain-google-vertexai;: Installed. No version info available.
> langchain-groq;: Installed. No version info available.
> langchain-huggingface;: Installed. No version info available.
> langchain-mistralai;: Installed. No version info available.
> langchain-ollama;: Installed. No version info available.
> langchain-openai;: Installed. No version info available.
> langchain-perplexity;: Installed. No version info available.
> langchain-text-splitters<1.0.0,>=0.3.8: Installed. No version info available.
> langchain-together;: Installed. No version info available.
> langchain-xai;: Installed. No version info available.
> langchain<1.0.0,>=0.3.23: Installed. No version info available.
> langsmith-pyo3: Installed. No version info available.
> langsmith<0.4,>=0.1.125: Installed. No version info available.
> langsmith<0.4,>=0.1.17: Installed. No version info available.
> numpy<3,>=1.26.2: Installed. No version info available.
> numpy>=1.26.0;: Installed. No version info available.
> numpy>=2.1.0;: Installed. No version info available.
> openai-agents: Installed. No version info available.
> openai<2.0.0,>=1.68.2: Installed. No version info available.
> opentelemetry-api: 1.31.1
> opentelemetry-exporter-otlp-proto-http: Installed. No version info available.
> opentelemetry-sdk: 1.31.1
> orjson: 3.10.16
> packaging: 24.2
> packaging<25,>=23.2: Installed. No version info available.
> pydantic: 2.11.2
> pydantic-settings<3.0.0,>=2.4.0: Installed. No version info available.
> pydantic<3.0.0,>=2.5.2;: Installed. No version info available.
> pydantic<3.0.0,>=2.7.4: Installed. No version info available.
> pydantic<3.0.0,>=2.7.4;: Installed. No version info available.
> pymilvus: 2.5.8
> pytest: Installed. No version info available.
> PyYAML>=5.3: Installed. No version info available.
> qdrant-client: 1.14.2
> requests: 2.32.3
> requests-toolbelt: 1.0.0
> requests<3,>=2: Installed. No version info available.
> rich: 14.0.0
> SQLAlchemy<3,>=1.4: Installed. No version info available.
> tenacity!=8.4.0,<10,>=8.1.0: Installed. No version info available.
> tenacity!=8.4.0,<10.0.0,>=8.1.0: Installed. No version info available.
> tiktoken<1,>=0.7: Installed. No version info available.
> types-pyyaml: 6.0.12.20250402
> typing-extensions>=4.7: Installed. No version info available.
> zstandard: 0.23.0

Labels: investigate
State: closed