Union of two layers may contain features with empty geometries

[Ari Jolma]

A union of a layer with one polygon and a layer with one point within the polygon results in a layer with three features: point, polygon, and an empty geometrycollection. Should the geometrycollection be discarded from the result?

[Even Rouault]

r25427 ogr/ogrsf_frmts/generic/ogrlayer.cpp ../autotest/ogr/ogr_layer_algebra.py -m "OGRLayer: Identity(), Union() and Intersect(): avoid creating features with null or empty geometry; when intersecting 2 polygons, do not consider a point or linear intersection as a valid one (#4772)"

[Even Rouault]

r25428 ogr/ogrsf_frmts/generic/ogrlayer.cpp ../autotest/ogr/ogr_layer_algebra.py -m "OGRLayer: Update(): avoid creating features with null or empty geometry (#4772)"