SplitShardCmd.checkDiskSpace can fail for transient core

SplitShardCmd.checkDiskSpace uses Solr's remote Metrics API through SolrClientNodeStateProvider to fetch the size of a core it wants to consider splitting. But if this occurs during a race in which the core is unloaded, there won't be such a metric, resulting in a null then an exception stating "missing index size information for parent shard leader".