Fix error message on empty title

modules/contest.js

@@ -87,7 +87,7 @@ app.post('/contest/:id/edit', async (req, res) => {
       contest.ranklist_id = ranklist.id;
     }
 
-    if (!req.body.trim()) throw new ErrorMessage('比赛名不能为空。');
+    if (!req.body.title.trim()) throw new ErrorMessage('比赛名不能为空。');
     contest.title = req.body.title;
     if (!Array.isArray(req.body.problems)) req.body.problems = [req.body.problems];
     contest.problems = req.body.problems.join('|');

modules/discussion.js

@@ -120,6 +120,7 @@ app.post('/article/:id/edit', async (req, res) => {
       if (!await article.isAllowedEditBy(res.locals.user)) throw new ErrorMessage('您没有权限进行此操作。');
     }
 
+    if (!req.body.title.trim()) throw new ErrorMessage('标题不能为空。');
     article.title = req.body.title;
     article.content = req.body.content;
     article.update_time = time;

modules/problem.js

@@ -256,7 +256,7 @@ app.post('/problem/:id/edit', async (req, res) => {
       }
     }
 
-    if (!req.body.trim()) throw new ErrorMessage('题目名不能为空。');
+    if (!req.body.title.trim()) throw new ErrorMessage('题目名不能为空。');
     problem.title = req.body.title;
     problem.description = req.body.description;
     problem.input_format = req.body.input_format;
@@ -345,7 +345,7 @@ app.post('/problem/:id/import', async (req, res) => {
 
     if (!json.success) throw new ErrorMessage('题目加载失败。', null, json.error);
 
-    if (!json.obj.trim()) throw new ErrorMessage('题目名不能为空。');
+    if (!json.obj.title.trim()) throw new ErrorMessage('题目名不能为空。');
     problem.title = json.obj.title;
     problem.description = json.obj.description;
     problem.input_format = json.obj.input_format;