Modify ARN toString to print a valid ARN when there's no region or acountId

[Madrigal]

Fixes #2416

Motivation and Context

Certain ARNs, like S3 or IAM, don't have to include region or accountId. This constructs a valid ARN

Arn.builder()
    .partition("aws")
    .service("s3")
    .resource("bucket")
    .build();

However, when toString() is called in this method, region and account are printed as literal null' so the ARN becomes

arn:aws:s3:null:null:bucket

instead of the valid

arn:aws:s3:::bucket

This causes the problem that ARNs produced by toString cannot be used as identifiers when passed to other services,since this ARN is not valid.

Modifications

Don't print null on Arn.toString() if region or accountId are not set.

Testing

Added unit tests to verify this behavior. Additionally, this has the nice property that the ARNs produced by toString() can be parsed by Arn.fromString().

Screenshots (if appropriate)

N/A

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist

• I have read the CONTRIBUTING document
• Local run of mvn install succeeds
• My code follows the code style of this project
• My change requires a change to the Javadoc documentation
• I have updated the Javadoc documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed
• I have added a changelog entry. Adding a new entry must be accomplished by running the scripts/new-change script and following the instructions. Commit the new file created by the script in .changes/next-release with your changes.
• My change is to implement 1.11 parity feature and I have updated LaunchChangelog

License

• I confirm that this pull request can be released under the Apache 2 license

[sonarqubecloud]

Quality Gate passed

Issues
5 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud