fix(tenant): 修复多租户定时任务执行结果为空时的异常

- 针对执行结果为 null 的情况增加了日志记录
- 为了避免 ConcurrentHashMap put空指针异常，将 result 设置为空字符串

yudao-framework/yudao-spring-boot-starter-biz-tenant/src/main/java/cn/iocoder/yudao/framework/tenant/core/job/TenantJobAspect.java

@@ -50,6 +50,12 @@ public class TenantJobAspect {
             TenantUtils.execute(tenantId, () -> {
                 try {
                     Object result = joinPoint.proceed();
+                    if(null == result){
+                        XxlJobHelper.log(StrUtil.format("[多租户({}) 执行任务({})，结果为：null]",
+                                tenantId, joinPoint.getSignature()));
+                        //避免ConcurrentHashMap put空指针异常
+                        result = "";
+                    }
                     results.put(tenantId, StrUtil.toStringOrEmpty(result));
                 } catch (Throwable e) {
                     results.put(tenantId, ExceptionUtil.getRootCauseMessage(e));