fix regression injected by #1263: (#1272)
fix regression injected by #1263: (#1272)

We must log a warning message if there is no admin user,
that means if the result of `kuzzle.internalEngine.bootstrap.adminExists()` is falsy.
kuzzleio/kuzzle,kuzzleio/kuzzle,kuzzleio/kuzzle,kuzzleio/kuzzle