修复账单如果没有附件则现实错误页面的问题

application/em/model/EmInvoice.php

@@ -118,7 +118,7 @@ class EmInvoice extends Base
 
         //查询抄表信息
         $uModel = Useage::get(['useage_id'=>$eiu->useage_id]);
-        if($uModel && $uModel->cert_id != ''){
+        if(isset($uModel->cert_id) && $uModel->cert_id != ''){
             $useage = Useage::with('photo')->get(['useage_id'=>$eiu->useage_id]);
             if($useage->photo_id) {
                 $receipt = Receipt::get(['id'=>$useage->photo_id]);