fix: web reader tool retrieve content empty (#957)

api/core/tool/web_reader_tool.py

             texts = character_splitter.split_text(page_contents)
             docs = [Document(page_content=t) for t in texts]
 
+            if len(docs) == 0:
+                return "No content found."
+
             docs = docs[1:]
 
             # only use first 5 docs