Add null check on core info variables in IndexSizeTrigger

A minor fix, but we've seen NPEs from IndexSizeTrigger when ZK is restarted since it's unable to fetch the core info. All we need is a null check.

In the patch I'll also update a string value: https://github.com/apache/lucene-solr/blob/branch_8x/solr/core/src/java/org/apache/solr/cloud/autoscaling/IndexSizeTrigger.java#L339

And might add a log to report index size when splits are enqueued