use current directory if none is supplied in InitializeParams

[GeneTinderholm]

I was just starting to play around with nim (seems like a cool language).

The language server was dying when trying to run with nvim, which made going through some examples from the docs and playing around with the code kinda rough. When working on the local version of the language server, it did work however, which was weird.

I did some spelunking and it looks like the rootUri field of InitializeParams is empty sometimes? I'm not super knowledgeable about language servers. Potentially this is because it doesn't detect any sort of nimble setup in the directory?

This change made most functionality work on my end, mostly auto-complete and hover, and doesn't seem like it would break things for anyone else.

[nazgulsenpai]

nimlangserver didn't work in helix editor for me ('nimlangserver has exited' immediately), so did nimble install https://github.com/GeneTinderholm/nimlangserver and this PR fixed it for me. Thank you and hopefully this gets merged.