修复联动调用选择空时的错误

application/admin/controller/Ajax.php

@@ -273,18 +273,18 @@ class Ajax extends Backend
     {
         $params = $this->request->get("row/a");
         if (!empty($params)) {
-            $province = isset($params['province']) ? $params['province'] : '';
-            $city = isset($params['city']) ? $params['city'] : '';
+            $province = isset($params['province']) ? $params['province'] : null;
+            $city = isset($params['city']) ? $params['city'] : null;
         } else {
-            $province = $this->request->get('province', '');
-            $city = $this->request->get('city', '');
+            $province = $this->request->get('province');
+            $city = $this->request->get('city');
         }
         $where = ['pid' => 0, 'level' => 1];
         $provincelist = null;
-        if ($province !== '') {
+        if ($province !== null) {
             $where['pid'] = $province;
             $where['level'] = 2;
-            if ($city !== '') {
+            if ($city !== null) {
                 $where['pid'] = $city;
                 $where['level'] = 3;
             }