Migrate from Commons Lang to native Java Platform functionality

[timja]

No need to use a third-party library when this functionality is available in the Java Platform.

Part of the effort to remove Commons Lang 2 from Jenkins core — jenkinsci/jenkins#16404,
jenkinsci/jenkins#26105. Commons Lang 2 is EOL and carries an unfixed advisory
(GHSA-j288-q9x7-2f5v).

What's changed

• StringUtils.isEmpty / isNotEmpty → Util.fixEmpty(x) == null / != null.
• StringUtils.join(messages, "\n\n") → String.join("\n\n", messages) in both log parsers.
  LogNodeUtils.getMessages only inserts a message after checking message != null && !message.isEmpty(),
  so the list never holds nulls and String.join cannot NPE here.

No new dependency is added. commons-lang3-api is not an option here: it requires core 2.479.3 and
this plugin's baseline is 2.361.4.

Note on testing: this plugin does not build on JDK 21 — master fails the same way, with
IllegalArgumentException: Unsupported class file major version 65 out of the Groovy that parent
POM 4.65 pulls in. I verified on JDK 17 instead. That is pre-existing and untouched by this PR.

Testing done

mvn -B -ntp clean verify passes locally on Java 17 / macOS.
The ban-commons-lang-2 enforcer rule was left disabled because it needs parent POM
6.2116.v7501b_67dc517 or newer; bumping the parent from 4.65 pulls in the Jakarta EE 9
migration, which is out of scope for this change.

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

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🤖 This pull request was generated with AI assistance (Claude Code) as part of a bulk migration
across Jenkins plugins. If anything here looks wrong, please comment on this PR or contact
@parameter.