add "failIfEmptyCores" option for HealthCheckHandler

Background

We are hosting Solr on AWS EC2.
We have set up Solr to start in systemd and create a core at startup. (The replica type is "pull".)
I manage those EC2 instances in the AWS AutoScaling Group and use TargetGroup to access Solr via the Application Load Balancer.
For health checks in TargetGroup, we use the requireHealthyCores option using /api/node/health (henceforth called HealthCheckHandler).

In most cases this is not a problem.
However, this can cause additional problems with a rare core creation failure issue at startup.
If you use the requireHealthyCores option for HealtchCheckHandler, the status will be OK if the core is not there in the first place, so the health check will be OK even though the core is not present (an unusual condition).

Proposal

Therefore, we propose to add an option to set the status to Fail if the core does not exist. That way, we can make the health check fail even if the core was not created for some reason.

We believe this option should only work if the requireHealthyCores option is true.

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