fix(extensions-agui): update condition check for StringBuilder length

Author: lynx009

agentscope-extensions/agentscope-extensions-agui/src/main/java/io/agentscope/core/agui/adapter/AguiAgentAdapter.java

@@ -325,14 +325,14 @@ private String extractToolResultText(ToolResultBlock toolResult) {
         StringBuilder sb = new StringBuilder();
         for (ContentBlock output : toolResult.getOutput()) {
             if (output instanceof TextBlock textBlock) {
-                if (sb.length() > 0) {
+                if (!sb.isEmpty()) {
                     sb.append("\n");
                 }
                 sb.append(textBlock.getText());
             }
         }
 
-        return sb.length() > 0 ? sb.toString() : null;
+        return !sb.isEmpty() ? sb.toString() : null;
     }
 
     /**