fix recommendClassList empty bug

app/service/course/v5/report.js

@@ -281,7 +281,7 @@ class ReportService extends Service {
 
     // 按照分数降序，价格升序的顺序排列
     classList = _.orderBy(classList, [ 'score', 'filter_price' ], [ 'desc', 'asc' ]);
-    classList = classList.slice(0, 3);
+    classList = ctx.isEmpty(classList) ? [] : classList.slice(0, 3);
 
     for (let i = 0; i < classList.length; i++) {
       classList[i].match = 99 - i * 3 - Math.floor(Math.random() * 3);