git: add default value null to programs.git.signing.key

[folliehiyuki]

Description

Right now, if other signing options are defined without key specified, home-manager switch will fail.

Checklist

• [x] Change is backwards compatible.

• [x] Code formatted with ./format.

• [x] Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.

• [ ] Test cases updated/added. See example.

• [x] 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

  • [ ] Added myself as module maintainer. See example.

Maintainer CC

@rycee