LAPIS readiness probe should return ready even on empty database

[corneliusroemer]

It seems that the LAPIS readiness probe fails if the database is empty. We should use a readiness probe that works for an empty database.

Maybe #2934 can help here? @fengelniederhammer

[theosanderson]

Are you sure? I thought we addressed that. We have lots of instances that show 0 results.

[corneliusroemer]

Hmm, it's possible I'm wrong here - I had definitely forgotten to start the backend, so that might have contributed, though it's odd that LAPIS showed not ready in that case instead of SILO preprocessing.

All I know is that LAPIS was showing not ready for a long time in k9s, while the backend was not up. Let's see.