Only show Retry Args vertical scrollbar if needed.

It's only cosmetic, but disabled scrollbars were appearing when not needed and adding clutter.

![](https://www.evernote.com/shard/s1/sh/8da8e66c-2601-477b-bd24-a4f798c5d309/8bdd9355d34ee7f9de7965ccef7f866b/deep/0/Sidekiq.png)