reduce unneed query when post without attachements.
reduce unneed query when post without attachements.

This commit reduce following query:

```
 MediaAttachment Load (0.9ms)  SELECT "media_attachments".* FROM "media_attachments" WHERE "media_attachments"."status
_id" IS NULL AND 1=0 ORDER BY id asc
```

pixiv/mastodon,pixiv/mastodon,pixiv/mastodon,pixiv/mastodon