community[patch]: bug fix - add empty metadata when metadata not provided (langchain-ai#17669)

Code fix to include empty medata dictionary to aadd_texts if metadata is
not provided.

Author: lalanikarim

libs/community/langchain_community/vectorstores/surrealdb.py

@@ -116,6 +116,8 @@ async def aadd_texts(
             data = {"text": text, "embedding": embeddings[idx]}
             if metadatas is not None and idx < len(metadatas):
                 data["metadata"] = metadatas[idx]  # type: ignore[assignment]
+            else:
+                data["metadata"] = []
             record = await self.sdb.create(
                 self.collection,
                 data,