NIFI-13160 Correct frontend changes evaluation in ci-workflow

[exceptionfactory]

Summary

NIFI-13160 Corrects evaluation of the steps.changes.outputs.frontend variable in the GitHub ci-workflow when setting the frontend.skipTests build property. The initial implementation always set the frontend.skipTests value to false. The updated approach follows ternary example from GitHub Actions Expressions documentation.

Tracking

Please complete the following tracking steps prior to pull request creation.

Issue Tracking

• Apache NiFi Jira issue created

Pull Request Tracking

• Pull Request title starts with Apache NiFi Jira issue number, such as NIFI-00000
• Pull Request commit message starts with Apache NiFi Jira issue number, as such NIFI-00000

Pull Request Formatting

• Pull Request based on current revision of the main branch
• Pull Request refers to a feature branch with one commit containing changes

Verification

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Build

• Build completed using mvn clean install -P contrib-check
  • JDK 21

UI Contributions

• NiFi is modernizing its UI. Any contributions that update the current UI also need to be implemented in the new UI.

Licensing

• New dependencies are compatible with the Apache License 2.0 according to the License Policy
• New dependencies are documented in applicable LICENSE and NOTICE files

Documentation

• Documentation formatting appears as expected in rendered files