Update observed rules: add holiday removal support

[arkid15r]

Proposed change

This PR extends our ObservedRule class to support "move to None" cases. In other words it's a holiday removal (no holiday + no observance type of rule). I've got this idea while reviewing https://github.com/vacanza/python-holidays/pull/1792, specifically from this comment. It'll be useful for making IFEU holiday calendar a bit cleaner.

Let me know your thoughts on this when you get a chance.

P.S. I did my best to avoid naming the rule ???_TO_NONDAY :)

Type of change

• [ ] New country/market holidays support (thank you!)
• [ ] Supported country/market holidays update (calendar discrepancy fix, localization)
• [x] Existing code/documentation/test/process quality improvement (best practice, cleanup, refactoring, optimization)
• [ ] Dependency update (version deprecation/upgrade)
• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] Breaking change (a code change causing existing functionality to break)
• [ ] New feature (new python-holidays functionality in general)

Checklist

• [x] I've followed the contributing guidelines
• [x] I've run make pre-commit, it didn't generate any changes
• [x] I've run make test, all tests passed locally

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.1% Duplication on New Code

See analysis details on SonarCloud

[coveralls]

Pull Request Test Coverage Report for Build 9130851431

Details

• 18 of 18 (100.0%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 9099445825:	0.0%
Covered Lines:	11169
Relevant Lines:	11169

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💛 - Coveralls