Fix NoneType has no len() in DDG tool (langchain-ai#3334)

Per
https://github.com/deedy5/duckduckgo_search/blob/46ac914daa614843cfa2ee3dd4663a5862e775a2/duckduckgo_search/ddg.py#L109,
ddg function actually returns None when there is no result.

Author: Dianliang233

langchain/utilities/duckduckgo_search.py

@@ -49,7 +49,7 @@ def run(self, query: str) -> str:
             time=self.time,
             max_results=self.max_results,
         )
-        if len(results) == 0:
+        if results is None or len(results) == 0:
             return "No good DuckDuckGo Search Result was found"
         snippets = [result["body"] for result in results]
         return " ".join(snippets)
@@ -77,7 +77,7 @@ def results(self, query: str, num_results: int) -> List[Dict]:
             max_results=num_results,
         )
 
-        if len(results) == 0:
+        if results is None or len(results) == 0:
             return [{"Result": "No good DuckDuckGo Search Result was found"}]
 
         def to_metadata(result: Dict) -> Dict: