Improve robustness and logging for Null Pointer handling in Docker.GC task

An NPE can occur within the Docker.GC task at https://github.com/sonatype/nexus-internal/blob/main/private/plugins/nexus-repository-docker/src/main/java/org/sonatype/nexus/repository/docker/internal/datastore/recipe/DockerGCFacetImpl.java#L230

This can happen when the docker_assets.attributes DB field for a given Docker asset is empty. 

Expected:

The possibility of an NPE occurring should be handled and if one does occur, for the sake of troubleshooting, appropriate logging (ERROR) should be added indicating the ID/details of the affected asset.