Change WebBaseLoader metadata parsing to set missing metadata to descriptive string instead of `None` (#8175)

Solves #8174 & #3542

Co-authored-by: mevans <mevans@palantir.com>

libs/langchain/langchain/document_loaders/web_base.py

@@ -30,9 +30,9 @@ def _build_metadata(soup: Any, url: str) -> dict:
     if title := soup.find("title"):
         metadata["title"] = title.get_text()
     if description := soup.find("meta", attrs={"name": "description"}):
-        metadata["description"] = description.get("content", None)
+        metadata["description"] = description.get("content", "No description found.")
     if html := soup.find("html"):
-        metadata["language"] = html.get("lang", None)
+        metadata["language"] = html.get("lang", "No language found.")
     return metadata