SOLR-16343: Handle MDC snapshot being null

[dee42]

https://issues.apache.org/jira/browse/SOLR-16343

Description

With SLF4J 2, MDC context can be null. We need to handle this.

Solution

Ignore the snapshot if it is null

Tests

I tested it fixed my traceback as recorded in the JIRA ticket.

Checklist

Please review the following and check all that apply:

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
• I have given Solr maintainers access to contribute to my PR branch. (optional but recommended)
• I have developed this patch against the main branch.
• I have run ./gradlew check.
• I have added tests for my changes.
• I have added documentation for the Reference Guide

[risdenk]

I checked the Javadoc for https://www.slf4j.org/api/org/slf4j/MDC.html#getCopyOfContextMap-- and says it can return null.

[risdenk]

Merged in ae242ff - Thanks @dee42