Why are null geometries not allowed?

[McSurf84]

The stac-spec says that the geometry of an item may also be null - see stac-spec.

However, the migration for the database sets the geometry field to NOT NULL at pgstac.0.9.1.sql#L2030 and it is therefore not possible to create items with a zero geometry.

Does this have a deeper meaning or why is NULL not allowed?

If this is not intention, I would create a pull request to change this.