Allow `null` geometries for Items

[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.

[bitner]

I believe that earlier in the stac spec, geometry may have been required.

I'm happy to take a PR to remove that constraint, but I would warn that it may be more work that just removing that constraint as some of the transformations to and from the table layout to json may be making the assumption that a geometry always exists.

[vprivat-ads]

I ran the tests without the NULL constraint and no error is reported: https://github.com/vprivat-ads/pgstac/pull/1/checks

But I guess we are far from having a complete coverage?

[spinto]

Any update about this issue? We have some STAC data for which "geometry" is not available, so it would be good if we can insert with PgSTAC STAC items with null geometry metadata.

[vincentsarago]

FYI in stac-fastapi I've added a check to prevent users to load Items without geometry stac-utils/stac-fastapi-pgstac#259 But we should fix this to comply to the stac spec