operator backup with preexisting pv does not work

Feature from THREESCALE-5312

does not work if pre-existing persistentvolume is used. Operator tries to create pvc from that pv, but size definition resources.requests.storage is missing and attempt to create pvc fails. reproducible also manually:

$ echo 'apiVersion: v1
kind: PersistentVolumeClaim
metadata:
{{ name: test-qe-examplepvc}}
spec:
{{ accessModes:}}
{{ - ReadWriteOnce}}
{{ volumeName: "mypreexistingvolume"' | oc create -f -}}
The PersistentVolumeClaim "test-qe-examplepvc" is invalid: spec.resources[storage]: Required value