## **Description:**
When using the Tavily retriever with include_raw_content=True, the
retriever occasionally fails with a Pydantic ValidationError because
raw_content can be None.
The Document model in langchain_core/documents/base.py requires
page_content to be a non-None value, but the Tavily API sometimes
returns None for raw_content.
This PR fixes the issue by ensuring that even when raw_content is None,
an empty string is used instead:
```python
page_content=result.get("content", "")
if not self.include_raw_content
else (result.get("raw_content") or ""),
8977ac5ab0