Running "docker delete unused manifests and images" task when blob storage is not available deletes all layers

Running a "docker delete unused manifests and images" when a repository's blob storage is not available causes all docker layers in the target repository to be removed.

Expected:  That task should not remove layers simply because their underlying blobs are missing.   

2020-07-28 11:47:51,909-0500 INFO [quartz-7-thread-4] *SYSTEM org.sonatype.nexus.repository.docker.internal.orient.DockerGCFacetImpl - Garbage collection starting on repository: RepositoryImpl$$EnhancerByGuice$$6e46ea48

Unknown macro: {type=hosted, format=docker, name='docker-hosted'}

2020-07-28 11:47:51,999-0500 WARN [quartz-7-thread-4] *SYSTEM org.sonatype.nexus.blobstore.file.FileBlobStore - Attempt to access non-existent blob a116eb31-cb5e-4a4d-9686-883430f2409c (/Users/rseddon/temp/sonatype-work/nexus3/blobs/default/content/vol-21/chap-42/a116eb31-cb5e-4a4d-9686-883430f2409c.properties)
2020-07-28 11:47:52,002-0500 WARN [quartz-7-thread-4] *SYSTEM org.sonatype.nexus.repository.docker.internal.orient.DockerGCFacetImpl - Unable to read V2 Manifest for asset Asset{metadata=AttachedEntityMetadata{schema=asset, document=asset#40:0

Unknown macro: {bucket}

v1}, name=v2/hello-world/manifests/latest}, Blob default@7137D4AF-C1807AC6-A7F17383-6F1AE542-61E4E66A:a116eb31-cb5e-4a4d-9686-883430f2409c exists in metadata, but is missing from the blobstore
2020-07-28 11:47:52,070-0500 WARN [quartz-7-thread-4] *SYSTEM org.sonatype.nexus.blobstore.file.FileBlobStore - Attempt to access non-existent blob 66d2241d-f033-4606-bcb8-2fa58835de54 (/Users/rseddon/temp/sonatype-work/nexus3/blobs/default/content/vol-24/chap-28/66d2241d-f033-4606-bcb8-2fa58835de54.properties)
2020-07-28 11:47:52,071-0500 WARN [quartz-7-thread-4] *SYSTEM org.sonatype.nexus.blobstore.file.FileBlobStore - Attempt to mark-for-delete non-existent blob 66d2241d-f033-4606-bcb8-2fa58835de54, hard deleting instead