Remove empty geometries for contested land zones

[irm-codebase]

Describe the bug
Some datasets (overture) may add an empty polygon for contested zones.
To avoid this case, the schema should always drop empty polygons and later validate all shapes.

Found in modelblocks-org/module_area_potentials#1

To Reproduce
Steps to reproduce the behaviour:

1. Run for china (with subtype: regions)
2. Check produced dataset. The first row will have empty shapes (contested with India).

Expected behaviour
These cases should be excluded to reflect our approach to marine regions.