Fix doc-str Guile

[DMClVG]

doc-str would silently fail and not display anything in nvim when used with Guile due to a missing check on the optional opts.on-result property. This PR fixes it

[Olical]

Thanks! This looks like a good fix. I would use a when instead of an if when it's a single branch though, I'll correct that post-merge happily because this is still fine Fennel, just would fail linting if such linting existed. (maybe it does!)

[DMClVG]

You're right. Force of habit lol