NIFI-14188 Fix NPE in FrameworkSslContextProvider

[exceptionfactory]

Summary

NIFI-14188 Fixes a potential NullPointerException in the FrameworkSslContextProvider when evaluating the nifi.security.keystoreType or nifi.security.truststoreType properties. The solution returns a default empty string when the property value is null, avoiding the NPE on the subsequent comparison.

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 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 mvn clean install -P contrib-check
  • JDK 21

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

[joewitt]

looks good - running local build to confirm w contrib checks since will be done faster than gh then will merge if clean