pkg/operator: only set storage class annotation if given
pkg/operator: only set storage class annotation if given

When the value of the annotation is empty the cluster still tries to
auto provision the PVC/PV.

sjuxax/prometheus-operator,mmerrill3/prometheus-operator,brancz/prometheus-operator,eedugon/prometheus-operator,mmerrill3/prometheus-operator,mmerrill3/prometheus-operator,gianrubio/prometheus-operator,jescarri/prometheus-operator,jescarri/prometheus-operator,coreos/prometheus-operator,gianrubio/prometheus-operator,coreos/prometheus-operator,nirdothan/prometheus-operator,eedugon/prometheus-operator,nirdothan/prometheus-operator,gianrubio/prometheus-operator,brancz/prometheus-operator,brancz/prometheus-operator,sjuxax/prometheus-operator