Description of problem:
After creating a volumesnapshot instance without finding the PersistentVolumeSource, it could not be deleted anymore.

Version-Release number of selected component (if applicable):
[wduan@MINT snapshot]$ oc get clusterversion
NAME      VERSION                             AVAILABLE   PROGRESSING   SINCE   STATUS
version   4.6.0-0.nightly-2020-08-06-131904   True        False         14h     Cluster version is 4.6.0-0.nightly-2020-08-06-131904

How reproducible:
2/2

Steps to Reproduce:
0. aws-ebs-csi-driver is installed by default in 4.6 
1. Create pvc with in-tree sc gp2 
$ oc get pvc
NAME       STATUS   VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS   AGE
mypvc-1    Bound    pvc-40497255-b302-45e3-953a-aff57acba5bc   1Gi        RWO            gp2            8m55s

2. Create volumesnapshotclass 
apiVersion: snapshot.storage.k8s.io/v1beta1
kind: VolumeSnapshotClass  
metadata:
  name: csi-aws-ebs-snapclass
  annotations:
    snapshot.storage.kubernetes.io/is-default-class: "true"
driver: ebs.csi.aws.com
deletionPolicy: Delete

3. Create volumesnapshot with snapshotclass, instance is in ReadToUse=false
apiVersion: snapshot.storage.k8s.io/v1beta1
kind: VolumeSnapshot
metadata:
  name: mysnapshot-1
spec:
  volumeSnapshotClassName: csi-aws-ebs-snapclass
  source:
    persistentVolumeClaimName: mypvc-1

$ oc get volumesnapshot
NAME           READYTOUSE   SOURCEPVC   SOURCESNAPSHOTCONTENT   RESTORESIZE   SNAPSHOTCLASS           SNAPSHOTCONTENT   CREATIONTIME   AGE
mysnapshot-1   false        mypvc-1                                           csi-aws-ebs-snapclass                                    8m41s

4. Detele the volumesnapshot instance
$ oc delete volumesnapshot mysnapshot-1
volumesnapshot.snapshot.storage.k8s.io "mysnapshot-1" deleted

^C
[wduan@MINT snapshot]$ oc get volumesnapshot
NAME           READYTOUSE   SOURCEPVC   SOURCESNAPSHOTCONTENT   RESTORESIZE   SNAPSHOTCLASS           SNAPSHOTCONTENT   CREATIONTIME   AGE
mysnapshot-1   false        mypvc-1                                           csi-aws-ebs-snapclass                                    91m


Actual results:
This volumesnapshot instance could not be deleted.

Expected results:
This volumesnapshot instance should be deleted.

Master Log:

Node Log (of failed PODs):

PV Dump:

PVC Dump:

StorageClass Dump (if StorageClass used by PV/PVC):

Additional info: