Implemented `Feature::set_field_null`

[metasim]

• [X] I agree to follow the project's code of conduct.

• [X] I added an entry to CHANGES.md if knowledge of this change could be valuable to users.

Closes #427 Credit to @nms-scribe

[lnicola]

I was too afraid to check in #427, looked at the docs now, but still have no idea. Does anyone know the difference between OGR_F_SetFieldNull and OGR_F_UnsetField?

Does the latter actually remove the field value in formats like GeoJSON?

[metasim]

Docs:

• OGR_F_SetFieldNull: Clear a field, marking it as null.
• OGR_F_UnsetField: Clear a field, marking it as unset.

@lnicola Yeah, I don't understand the semantics either.

Maybe we just mark this as "draft" and come back to it later?

[lnicola]

No, I would have merged it, but unsetting all fields of all features in all layers seems a bit excessive. But it's only 21 features, so feel free to merge :slightly_smiling_face:.

[metasim]

I simplified the test.