fix: only initialize default namespace when given

[johannesfrey]

Otherwise the controller is not able to delete the vcluster.

Error message:

2024-06-13T09:15:34Z    ERROR   Reconciler error        {"controller": "vcluster", "controllerGroup": "infrastructure.cluster.x-k8s.io", "controllerKind": "VCluster", "VCluster": {"name":"vcluster","namespace":"vcluster"}, "namespace": "vcluster", "name": "vcluster", "reconcileID": "6f7c4b3b-e18c-4e48-8c9c-645ff95b708a", "error": "unable to list: vcluster because of unknown namespace for the cache"}