fix 在没有选择学生的情况下报错的bug (BUG #3931)

server-db/src/main/java/cn/exploring/engine/server/db/service/StudentInfoService.java

@@ -117,6 +117,10 @@ public class StudentInfoService {
                 .field(StudentVo::getCallTimes, maxCall).op(LessThan.class)
                 .build());
 
+        if (Objects.isNull(record)) {
+            return null;
+        }
+
         increaseCallTimeByPrimaryKey(record.getId());
         return record;
     }

server-wx-api/src/main/java/cn/exploring/engine/server/wx/web/StudentController.java

@@ -4,6 +4,7 @@ import cn.exploring.engine.server.core.util.ResponseUtil;
 import cn.exploring.engine.server.db.domain.StudentInfo;
 import cn.exploring.engine.server.db.domain.vo.StudentInfoExcelImportResultVo;
 import cn.exploring.engine.server.db.domain.vo.StudentInfoExcelImportVo;
+import cn.exploring.engine.server.db.domain.vo.StudentVo;
 import cn.exploring.engine.server.db.service.ClassService;
 import cn.exploring.engine.server.db.service.StudentInfoService;
 import com.google.common.collect.ImmutableMap;
@@ -17,6 +18,7 @@ import javax.annotation.Nullable;
 import javax.annotation.Resource;
 import java.util.List;
 import java.util.Objects;
+import java.util.Optional;
 
 @RestController
 @RequestMapping("/wx/student")
@@ -40,7 +42,8 @@ public class StudentController {
 
     @GetMapping("randomSelect")
     public Object randomSelect(@RequestParam Integer classId, @Nullable @RequestParam Integer maxCall) {
-        return ResponseUtil.ok(studentInfoService.randomSelectStudentInfo(classId, maxCall));
+        StudentVo select = studentInfoService.randomSelectStudentInfo(classId, maxCall);
+        return Optional.ofNullable(select).map(ResponseUtil::ok).orElse(ResponseUtil.fail(402, "未包含任何学生，请重新选择条件。"));
     }
 
     @GetMapping("pointList")