Only set commit-lockfile-summary when commit-msg is defined

[MattSturgeon]

Description

Fixes #210

Only pass --option commit-lockfile-summary commitMessage to nix flake update when the commit-msg is defined; i.e. non-empty.

Changed the input's default to "" to allow it to be empty

Checklist

• [x] Tested functionality against a test repository (see "How to test changes")
  • https://github.com/MattSturgeon/test-update-flake-lock
• [ ] Added or updated relevant documentation (leave unchecked if not applicable)
• [x] Updated the unit test with a new test case

Summary by CodeRabbit

• New Features

  • No default commit message is applied when running updates; if you don’t supply a commit message, none will be set.
  • The lockfile summary option is omitted when the commit message is empty, aligning behavior with user input.

• Tests

  • Added test coverage for the empty/omitted commit message scenario to ensure correct argument generation.