fix nil.year no method error in sync_conference_papers

lib/tasks/sync_ntnu_personal_data_tasks.rake

@@ -121,8 +121,14 @@ def sync_conference_papers(query_result, user)
       when '20100504082950644' # 使用語言
         wc.language = data['data'].first['v']
       when '20101012151243807' # 會議開始日期
-        wc.period_start_date = data['data'].first['v']
-        wc.year = wc.period_start_date.year
+        temp_date = Date.strptime(data['data'].first['v'], '%Y/%m/%d') rescue nil
+        if temp_date.nil?
+          wc.period_start_date = nil
+          wc.year = nil
+        else
+          wc.period_start_date = temp_date
+          wc.year = temp_date.year
+        end
       when '20101012151318602' # 會議結束日期
         wc.period_end_date = data['data'].first['v']
         #when '20190109101534767' # 合著之國際學者姓名與機構