Description of problem:

When using native csi/adatamover, Velero tries to cleanup the resources before marking the backup as complete, during this cleanup Velero is jitting a nil pointer dereference 

Logs:
I0830 19:17:12.833619 1 request.go:665] Waited for 1.043154119s due to client-side throttling, not priority and fairness, request: GET:
https://254.53.64.1:443/apis/authorization.openshift.io/v1?timeout=32s
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x1a8a24d]
goroutine 13553 [running]:
github.com/vmware-tanzu/velero/pkg/controller.(*backupController).deleteVolumeSnapshot.func1(0xc00241c640)
/go/src/github.com/vmware-tanzu/velero/pkg/controller/backup_controller.go:949 +0xad
created by github.com/vmware-tanzu/velero/pkg/controller.(*backupController).deleteVolumeSnapshot
/go/src/github.com/vmware-tanzu/velero/pkg/controller/backup_controller.go:945 +0xf7
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x188 pc=0x1a8a27a]
goroutine 13534 [running]:
github.com/vmware-tanzu/velero/pkg/controller.(*backupController).deleteVolumeSnapshot.func1(0xc002b383c0)
/go/src/github.com/vmware-tanzu/velero/pkg/controller/backup_controller.go:952 +0xda
created by github.com/vmware-tanzu/velero/pkg/controller.(*backupController).deleteVolumeSnapshot
/go/src/github.com/vmware-tanzu/velero/pkg/controller/backup_controller.go:945 +0xf7
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x188 pc=0x1a8a27a]
goroutine 13532 [running]:
github.com/vmware-tanzu/velero/pkg/controller.(*backupController).deleteVolumeSnapshot.func1(0xc00241d2c0)
/go/src/github.com/vmware-tanzu/velero/pkg/controller/backup_controller.go:952 +0xda
created by github.com/vmware-tanzu/velero/pkg/controller.(*backupController).deleteVolumeSnapshot
/go/src/github.com/vmware-tanzu/velero/pkg/controller/backup_controller.go:945 +0xf7

Version-Release number of selected component (if applicable):

How reproducible:

Steps to Reproduce:
1. Create a backup with bad backup CR config

```
apiVersion: velero.io/v1
kind: Backup
metadata:
name: cluster-pvcs
namespace: openshift-adp
spec:
csiSnapshotTimeout: 10m0s
defaultVolumesToRestic: false
includedNamespaces:

• '*'
  includedResources:
• PersistentVolumeClaim
  labelSelector:
  matchLabels:
  oadp-backup: 'true'
  storageLocation: openshift-backups-1
  ttl: 24h0m0s
  ```

   

Actual results:

Expected results:

Additional info: