Task splitting fails if flatgeobuf data extract is uploaded

[spwoodcock]

Describe the bug

• The task splitting algorithm works if a data extract is generated, or uploaded as geojson.
• The task splitting algorithm returns no features if a data extract is uploaded in flatgeobuf format.
  • The flatgeobuf successfully loads, is parsed as geojson and displayed on the map.
  • I assume an empty geojson (or the fgb file) is passed to the task_split endpoint, instead of the geojson generated from it.

To Reproduce

• Project creation with an uploaded flatgeobuf, attached.

Expected behavior

• The task splitting returns splits, not a geojson with no geometries.

Screenshots

Uploading the flatgeobuf works, displaying as custom_extract.geojson (after conversion):

After splitting:

Returned data:

[spwoodcock]

The PR fixes geojson parsing, plus cases with nested GeometryCollection.

Part if the issue was attempting to use a custom data extract containing only buildings, so there were not roads and rivers for splitting.

I will make another issue about this.