shards.info with shards.tolerant can yield an empty key

When using shards.tolerant=true and shards.info=true when a shard isn't available (and maybe other circumstances), the shards.info section of the response may have an empty-string key child with a value that is ambiguous as to which shard(s) couldn't be reached.

This problem can be revealed by modifying org.apache.solr.cloud.TestDownShardTolerantSearch#searchingShouldFailWithoutTolerantSearchSetToTrue to add shards.info and then examine the response in a debugger.