当我按照该节内容做完之后尝试提交空评论，这时服务器报错。

ActionView::Template::Error (undefined method `>' for nil:NilClass):

     9:       <h5 class="name"><%= link_to c.user.name, '#' %></h5>

    10:       <span class="datetime">

    11:         <%= time_ago_in_words c.created_at %>前

    12:       </span>

    13:     </div>

  app/views/shared/_comment.html.erb:11:in `_app_views_shared__comment_html_erb__1032379341_104080670'

  app/views/comments/create.js.erb:1:in `_app_views_comments_create_js_erb___822254942__633045858'

  app/controllers/comments_controller.rb:12:in `create'