neovimUtils: only set packpath/rtp as needed

[linsui]

... when there are plugins. It doesn't make sense to set rtp to an empty directory.

Description of changes

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.05 Release Notes (or backporting 22.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[teto]

did you have any issue with it or you are just annoyed to see a useless folder in packpath ?

I would like to make it possible to disable as part of #205932 (for instance it will be useless in home-manager with nix-community/home-manager#3717) so I will probably refactor that one way or another later.

[linsui]

you are just annoyed to see a useless folder in packpath ?

This.

IIUC, your PR makes it optional. But I thought it shouldn't be added to the startupCommands at all if the directory is empty.

[teto]

I will refactor this later. Thanks for contribution