Extend /healthz failure to empty metadata

### Is your proposal related to a problem?

Applying metadata can take many seconds, if not minutes. During that time, `/healthz` reports `200 OK`, even with `strict=true`, because nothing is inherently wrong with Hasura. But I want consumers to handle empty metadata the same way as broken metadata.

And by "empty metadata" I mean whichever state the metadata is in when Hasura is launched for the very first time.

### Describe the solution you'd like

Add another argument to `/healthz` that returns 500 if metadata is empty. For example `/healthz?strict=true&empty=true` would return 500 if metadata is either inconsistent or missing.

### Describe alternatives you've considered

Using `export_metadata` to check for `{}` (presumably) is not a great idea, because I'd have to do it on 200 OK, and our metadata is enormous when present.

Locate the /healthz handler and the existing strict=true behavior, then inspect how first-run metadata state is represented. Done means an empty=true query option returns 500 for empty or inconsistent metadata while preserving existing behavior otherwise, with tests covering the requested combinations.

Written by the indexing model from the issue text.