!245 修复后台分类调用PID为0时无法调用顶级分类

Merge pull request !245 from simon429/master

@@ -221,16 +221,15 @@ class Ajax extends Backend
         $pid = $this->request->get('pid');
         $pid = $this->request->get('pid');
         $where = ['status' => 'normal'];
         $where = ['status' => 'normal'];
         $categorylist = null;        
         $categorylist = null;        
-        if ($pid !== '') {
+        if ($pid || $pid === '0') {
+            $where['pid'] = $pid;
+        }
         if ($type) {
         if ($type) {
             $where['type'] = $type;
             $where['type'] = $type;
         }
         }
-            if ($pid) {
-                $where['pid'] = $pid;
-            }
         
         
         $categorylist = Db::name('category')->where($where)->field('id as value,name')->order('weigh desc,id desc')->select();
         $categorylist = Db::name('category')->where($where)->field('id as value,name')->order('weigh desc,id desc')->select();
-        }
+
         $this->success('', null, $categorylist);
         $this->success('', null, $categorylist);
     }
     }