`.zattrs` is not populated for one-level files

[vaxenburg]

Maybe I'm missing something but it looks like the population of .zattrs will not work for "shallow" source files that have this sort of one-level structure:

.
├── attributes.json
├── s0
├── s1
└── s2

I think the code will not consider the root zarr.hierarchy.Group and will never get past this line in that shallow case. So looks like the code assumes at least a two-level hierarchy?

[yuriyzubov]

Should be fixed now.