Improve output display of netCDF groups, when using `--only-diffs` option and when there are zero variables in a group

[danielfromearth]

Which problem is this feature request solving?

https://github.com/nasa/ncompare/issues/69#issuecomment-2532286741

Describe the solution you'd like

Describe alternatives you've considered

Can you submit a pull request?

Yes/No.