当用户尝试发送空消息时，会显示"消息不能为空"的错误提示。

client/mobile/components/Classroom/useClassroom.ts

@@ -506,6 +506,11 @@ export const useClassroom = ({ user }:{ user : User }) => {
   const sendMessage = async (): Promise<void> => {
     if (!imMessageManager.current || !classId) return;
 
+    if (!msgText.trim()) {
+      showToast('error', '消息不能为空');
+      return;
+    }
+
     try {
       await imMessageManager.current.sendGroupMessage({
         groupId: classId,