[import] Fails with no-entry .lift file of format exported from The Combine

[imnasnainaec]

Possibly related to #4159

The handling of empty LIFT files isn't robust.

To Reproduce

Steps to reproduce the behavior:

1. Export a project from The Combine
2. Unzip the file
3. Open the .lift file
4. Delete all the entries
5. Save the .lift file
6. Zip the folder
7. In The Combine, select the new zip for import
8. See this non-descript error:

400 https://qa-kube.thecombine.app/v1/projects/nonempty/lift/uploadandgetwritingsystems

This case should probably be handled, even though we don't allow exporting empty projects. Our export may be missing something that a blank export from FieldWorks has and our importer expects.