NIFI-12630 Fix NPE getLogger in ConsumeSlack and PublishSlack

[jrsteinebrey]

Summary

NIFI-12630

Tracking

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

Issue Tracking

• [X ] Apache NiFi Jira issue created

Pull Request Tracking

• [X ] Pull Request title starts with Apache NiFi Jira issue number, such as NIFI-00000
• [X ] 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

I added a new unit test to confirm the change works

Build

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

Licensing

No new files and no new dependencies

Documentation

No documentation changes

[mattyb149]

+1 LGTM, thanks for the fix! Merging to only main until #7673 is backported to support/nifi-1x.x