Only show the subreddit description label if there's a description

Hello everyone. Jumping in with something super small. This fixes #1137.
This is great but there's actually a simpler fix -- the docs don't mention these helpers because they're sorta deprecated, but we have an `{{rv}}` helper that is basically `{{rd}}` with a builting `{{if}}` for the given value.

So these two are equivalent:

```
{{#if public_description}}{{#rd "Description"}}

// same as
{{#rv "Description" val="public_description}}
```