[BUG] `volumes.csv` is empty when registering the right hempisphere only

**Describe the bug**
The `volumes.csv` only has headers if registration is run with the brain geometry set to "Right hemisphere"

**To Reproduce**
Register a right hemisphere sample.

**Expected behaviour**
The volumes should be calculated appropriately.

**Additional context**

See https://forum.image.sc/t/allen-atlas-not-matching-structures-file/118265 for details.
