JENKINS-68961: Fail gracefully on empty email address input

[shubh586]

Description

Fixes JENKINS-68961 by failing gracefully when an empty or whitespace-only email
address is provided. Previously, blank values were passed to JavaMail, resulting
in an AddressException stack trace. This change adds explicit validation to
reject blank input early with a clear, user-friendly error message.

Testing done

• Added a JUnit test (emptyAddressShouldFailGracefully) that verifies
  Mailer.stringToAddress fails gracefully when an empty email address is provided.
• Verified locally that the test fails without the fix and passes with the fix:
  mvn -Dtest=MailerTest#emptyAddressShouldFailGracefully test

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 (JENKINS-68961)
• Link to relevant pull requests, esp. upstream and downstream changes (not applicable)
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed