Add test for hasDayWithoutBirds method

[rabestro]

pull request

• Added a test to validate the behavior of hasDayWithoutBirds when the last day has zero visits.
• Updated the lastWeek field to be final and align with best practices.

https://forum.exercism.org/t/bird-watcher-in-java-should-we-add-a-test-case/17933

---

Reviewer Resources:

Track Policies

[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

If this PR does not affect the result of the test (or, for example, adds an edge case that is not worth rerunning all tests for), please add the following to the merge-commit message which will stops student's tests from re-running. Please copy-paste to avoid typos.

[no important files changed]

For more information, refer to the documentation. If you are unsure whether to add the message or not, please ping @exercism/maintainers-admin in a comment. Thank you!

[jagdish-15]

@kahgoh The forum topic doesn’t appear to have a clear resolution. What do you think, should this be merged with the [no important files changed] label, or not?

@rabestro Would love to hear your thoughts on this as well.