labels/.zgroup resolves despite no labels

[joshmoore]

This leads to the following output:

$ ome_zarr info http://localhost:8080/image/1.zarr/
http://localhost:8080/image/1.zarr/ [zgroup]
 - metadata
   - Multiscales
   - OMERO
 - data
   - (1, 1, 1, 512, 512)
not an ome-zarr: http://localhost:8080/image/1.zarr/labels/ [zgroup] (hidden)

for a simple import of a.fake.