Fix active_shards_percent_as_number NaN when total shard count is zero

[guluo2016]

In this PR I attempt to fix #134806.

Details see: #134806.

[cla-checker-service]

💚 CLA has been signed

[guluo2016]

I have already signed the CLA. If I missed anything, please let me know.

Please review, thanks a lot!

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)

[dakrone]

I think that perhaps we should represent this as 100%, rather than 0%, since the cluster would be totally healthy if there were no shards. 0% would indicate otherwise to me.

[DaveCTurner]

I don't think we should do this. If there are indices in the metadata but no routing table then the index is totally unassigned. The only way we can get here is if we skip over those indices here:

elasticsearch/server/src/main/java/org/elasticsearch/cluster/health/ClusterStateHealth.java

Lines 89 to 91 in ffe911b

	if (indexRoutingTable == null) {
	continue;
	}

That probably made sense prior to #33888 but these days it doesn't, the only time you can get here is in a freshly-started cluster, and for that we should be saying the health is red and the assigned percentage is 0.

I think we should drop that null check and instead make the constructor of org.elasticsearch.cluster.health.ClusterIndexHealth handle a null routing table by treating it as if all the shards are unassigned.

[guluo2016]

Thanks for the review @dakrone @DaveCTurner
I’ll update the code according to the review comments.

[guluo2016]

Please review, thanks!