tmux: make package nullable

[MaxWipfli]

Description

This makes the program.tmux.package argument nullable, allowing use of system tmux.

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -p treefmt nixfmt deadnix keep-sorted --run treefmt.

• Code tested through nix run .#tests -- test-all or
  nix-shell --pure tests -A run.all.

• Test cases updated/added. See example.

  • not required?

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • not applicable

• If this PR adds an exciting new feature or contains a breaking change.

  • not applicable