NIFI-15378 Remove empty lines from formatted XML in Content Viewer

[exceptionfactory]

Summary

NIFI-15378 Removes empty lines when rendering formatted XML in the standard Content Viewer.

The standard behavior of the JAXP Transformer is to add indentation and newlines regardless of the format of the input XML. As a result of this behavior, XML content with existing newlines is rendered with empty lines when selecting the Formatted option for XML in the Content Viewer.

Building on the discussion in PR #10679, the proposed changes implement filtering on the server for each line being written to the response stream. This approach avoids buffering large amounts of content in memory, and also avoids moving the newline replacement responsibility to the frontend.

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 contains commits signed with a registered key indicating Verified status

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

Please indicate the verification steps performed prior to pull request creation.

Build

• Build completed using ./mvnw clean install -P contrib-check
  • JDK 21
  • JDK 25

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