Empty geometry iso error for `vector_buffer` with negative buffer on small geometries

[soxofaan]

Open-EO/openeo-processes#404 added specification what should happen when negative buffering of small geometries with vector_buffer: use empty geometries.

Current implementation in openeo-python-driver however throws an error. Originally it was an obscure error message (see Open-EO/openeo-geopyspark-driver#314). The error was improved in #164 as quick fix.

Next step is to introduce empty geometries (and support them in subsequent processes like aggregate_spatial) instead of throwing errors.

[soxofaan]

(internal reference: MKTP-256)

[jdries]

fine for me, doesn't sound too hard

[soxofaan]

the most annoying part will probably be to make sure that all the downstream processes like aggregate_spatial, mask_polygon, apply_polygon, ... don't choke on these new empty geometries.