`oc-mirror` will hit error when use docker without namespace for OCI format mirror

Description of problem
`oc-mirror` will hit error when use docker without namespace for OCI format mirror

How reproducible:
always

Steps to Reproduce:
Copy the operator image with OCI format to localhost;
cat copy.yaml
apiVersion: mirror.openshift.io/v1alpha2
kind: ImageSetConfiguration
mirror:
operators:

• catalog: registry.redhat.io/redhat/redhat-operator-index:v4.11
  packages:
• name: multicluster-engine
  minVersion: '2.1.1'
  maxVersion: '2.1.2'
  `oc-mirror --config copy.yaml oci:///home/ocmirrortest/noo --use-oci-feature --oci-feature-action=copy --continue-on-error`
  Mirror the operator image with OCI format to registry without namespace :
  cat mirror.yaml
  apiVersion: mirror.openshift.io/v1alpha2
  kind: ImageSetConfiguration
  mirror:
  operators:
• catalog: oci:///home/ocmirrortest/noo/redhat-operator-index
  packages:
• name: multicluster-engine
  minVersion: '2.1.1'
  maxVersion: '2.1.2'

`oc-mirror --config mirror.yaml --use-oci-feature --oci-feature-action=mirror --dest-skip-tls docker://localhost:5000`

Actual results:
2. Hit error:
`oc-mirror --config mirror.yaml --use-oci-feature --oci-feature-action=mirror --dest-skip-tls docker://localhost:5000`
……
info: Mirroring completed in 30ms (0B/s)
error: mirroring images "localhost:5000//multicluster-engine/mce-operator-bundle@sha256:e7519948bbcd521390d871ccd1489a49aa01d4de4c93c0b6972dfc61c92e0ca2" is not a valid image reference: invalid reference format

Expected results:
2. No error

Additional info:
`oc-mirror --config mirror.yaml --use-oci-feature --oci-feature-action=mirror --dest-skip-tls docker://localhost:5000/ocmir` works well.