Don't fail for lack of internal registry if there are no internal images to copy

[sseago]

Currently the plugin fails a backup or restore if it can't locate the internal registry. This is done in the common backup/restore plugin, which means it will fail a migration for missing the registry even if there are no internal images to copy.

Running without a registry is a valid cluster configuration. The common plugin (which sets annotations for src and dest registry location/hostname) should just not set the annotation if it's not found rather than failing. Any plugins that assume this annotation exists will need to be modified accordingly. Basically, if it's not set on src, assume there are no internal images. If it's not set on dest but it was set on src, then fail the restore if there are internal images to copy from the src/migration registry, but don't fail if there are not.