SOLR-15099 Add null checks to IndexSizeTrigger

[megancarey]

…its are enqueued

Description

Want to avoid noisy NPE on core info variables since we already log.warn on line 330.

Solution

Minor fix: add null checks on the core info variables, as we've seen on ZK restarts that these are unavailable.

Tests

Ran IndexSizeTrigger test locally and it succeeded. Didn't add tests for this.

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 master branch.
• I have run ./gradlew check.
• I have added tests for my changes.
• I have added documentation for the Ref Guide (for Solr changes only).