Use "nil?" to judge project is deleted or not

What does this MR do and why?

Issue: https://jihulab.com/gitlab-cn/internal/content-security-backend/-/issues/42#note_4808318

What

当前在 Wiki 中上传的第一张图片不会被审核。

Why

代码中用 container.blank? 判断 ProjectWiki 是否存在（container 就是 ProjectWiki 对象），但当 ProjectWiki 存在且为空时，container.blank? 会返回 true，被误认为不存在。

How

将 container.blank? 改为 container.nil? 即可。

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