openshift-velero-plugin panics on imagestream backup, due to a missing secret

Description of problem:

• In a context where the backup and the `BackupStorageLocation` (BSL) are managed outside the scope of the `DataProtectionApplication` (DPA)), the OADP controller (i.e. DPAReconciler) does not [0] create the relevant [0] `oadp-$BSL-registry-secret`
• When the backup is run, the openshift-velero-plugin panics on the imagestream backup, with an `024-02-27T10:46:50.028951744Z time="2024-02-27T10:46:50Z" level=error msg="Error backing up item" backup=openshift-adp/$backupJobName error="error executing custom action (groupResource=imagestreams.image.openshift.io, namespace=$backedupNamespace, name=postgres): rpc error: code = Aborted desc = plugin panicked: runtime error: index out of range [1] with length 1, stack trace: goroutine 94 [...]` 

Steps to Reproduce:

1. Setup `backupImages: true` on the DPA
2. Create a Backup and a BackupStorageLocation that are not managed by the OADP controller (i.e. once without the `app.kubernetes.io/component": "bsl` label)
3. Run the backup and the velero logs for the above-mentioned panic error.

As a workaround to avoid the panic error, the customer can:

• Label the custom BSL with the relevant label

    $ oc label BackupStorageLocation $BSL app.kubernetes.io/component=bsl` 
  

• Once the BSL is labeled, wait until the DPA reconciles: 
    - NOTE: you can force this by doing any minor change to the DPA itself

• Once the DPA reconciles, confirm that the relevant `oadp-$BSL-registry-secret` has been created and that the right registry data has been populated into it:

    $ oc -n openshift-adp get secret/oadp-oadp-$BSL-registry-secret -o json | jq -r '.data'
  

Actual results:

• openshift-velero-plugin fails to backup imageStream and panics with a generic error:

  024-02-27T10:46:50.028951744Z time="2024-02-27T10:46:50Z" level=error msg="Error backing up item" backup=openshift-adp/$backupJobName error="error executing custom action (groupResource=imagestreams.image.openshift.io, namespace=$backedupNamespace, name=postgres): rpc error: code = Aborted desc = plugin panicked: runtime error: index out of range [1] with length 1, stack trace: goroutine 94 [...]` 
  

Expected results:

• Instead of throwing panic error, the expectation would be for the `openshift-velero-plugin` to print a human-readable error, highlighting the missing `oadp-$BSL-registry-secret`, as a requirement for the backup job to successfully run.

Additional information:

• This issue has been replicated with oadp-operator.v1.3.0

[0] https://github.com/openshift/oadp-operator/blob/oadp-1.3/controllers/registry.go#L610C1-L642C4