fix: toolCall.function.arguments maybe undefined (#2545)

当arg===undefined时，会导致 currentTool.function.arguments += arg; 字符串拼接出问题。

packages/service/core/workflow/dispatch/agent/runTool/toolChoice.ts

@@ -432,7 +432,7 @@ async function streamResponse({
       }
 
       /* arg 插入最后一个工具的参数里 */
-      const arg: string = toolCall?.function?.arguments;
+      const arg: string = toolCall?.function?.arguments ?? '';
       const currentTool = toolCalls[toolCalls.length - 1];
 
       if (currentTool) {