Fix for when recv returns nil

[paccator]

I was getting this error when connection can't be established:

NoMethodError: undefined method 'empty?' for nil
from .../lib/socksify/socksproxyable.rb:107:in 'Socksproxyable::InstanceMethodsConnect#socks_receive_reply'

I think it's related to a change in ruby 3.3:

BasicSocket#recv and BasicSocket#recv_nonblock returns nil instead of an empty string on closed connections.

https://rubyreferences.github.io/rubychanges/3.3.html#standard-library
https://bugs.ruby-lang.org/issues/19012

---

So here I propose to make sure we operate on a string, not nil.

In few places we could also just check for #nil? or empty? but #to_s in happy path is basically a noop.

Not much to add or change in tests.

[nix-ci-bot]

NixCI is ready to run on this PR.

[astro]

Personal opinion: I wouldn't merge this because I like the semantics of "" meaning no more data and nil signifying a closed read half.

However, I've passed maintainership to others a long time ago and will accept their decision.