fix: search engine did not return correctly (#1801)

lua/avante/config.lua

@@ -64,7 +64,7 @@ M._defaults = {
         },
         ---@type WebSearchEngineProviderResponseBodyFormatter
         format_response_body = function(body)
-          if body.answer_box ~= nil then return body.answer_box.result, nil end
+          if body.answer_box ~= nil and body.answer_box.result ~= nil then return body.answer_box.result, nil end
           if body.organic_results ~= nil then
             local jsn = vim
               .iter(body.organic_results)

lua/avante/llm_tools/init.lua

@@ -326,6 +326,7 @@ function M.web_search(opts, on_log)
     local jsn = vim.json.decode(resp.body)
     return search_engine.format_response_body(jsn)
   end
+  return nil, "Error: No search engine found"
 end
 
 ---@type AvanteLLMToolFunc<{ url: string }>