Fix NixOS setup when not using an Initial Session

[Lomig]

Thanks for your Greeter, it's a very nice tool!
I stumbled upon a couple of issues, though, so here is a small patch to fix that if you are so inclined ☺️

Context

• when not providing an initial_session option at configuration, the current flake defaults it to {}
  • depending to greetd version, it will search for the command key/value in this Attribute Set if present, leading to an error
• NixOS being sandboxed, sysc-greet will not have access to the standard directories to discover Session desktop files
  • the current version will show an empty Session list in this case

Description

• Don't expose a initial_session attribute set to greetd if none is provided to sysc-greet
• Add sand-boxed paths for Session discovery

Cheers!