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).

All usage is in Groovy sources. Six files:

• StringUtils.isEmpty / isNotEmpty / isBlank / isNotBlank → explicit null-and-empty checks;
  StringUtils.defaultIfEmpty(x, "") → Groovy's ?: elvis operator.
• ExceptionUtils.getStackTrace(e) → Functions.printThrowable(e), Jenkins core's own renderer.
• RandomStringUtils.random(10, true, true), which generates the Mac user account password, is now
  a local helper over SecureRandom. Commons Lang 2 backed that call with a plain java.util.Random,
  and I did not want to hand-write a weaker generator for a credential. Same length and alphabet as
  before; happy to drop it back if you would rather keep it identical.

Testing done

mvn -B -ntp clean verify passes locally on Java 11 / macOS. Java 21 does not work:
maven-license-plugin fails, on unmodified master too.
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.x was out of scope here.

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.