Fix empty groups tests

[nathaneastwood]

See inst/tinytest/test_empty_groups.R.

[nathaneastwood]

I am pausing the work on this until I get an answer to: https://twitter.com/nathaneastwood_/status/1376145804705021959?s=20

[nathaneastwood]

More information: https://stackoverflow.com/questions/66840953/how-does-dplyr-handle-empty-groups-when-displaying-groups-attribute

[nathaneastwood]

It would appear that the groups are created by recursively splitting: tidyverse/dplyr#5830