3.13.1 regression: "POINT ZM EMPTY".Union("POINT ZM EMPTY") returns "POINT EMPTY" instead of "MULTIPOINT ZM EMPTY"

[rouault]

Caught by GDAL autotest suite: https://github.com/OSGeo/gdal/actions/runs/13681199663/job/38265852044?pr=11924

41:     @pytest.mark.require_geos(3, 12)
41:     def test_ogr_geos_pointzm_empty():
41:     
41:         g1 = ogr.CreateGeometryFromWkt("POINT ZM EMPTY")
41:     
41:         g2 = g1.Union(g1)
41:     
41:         # GEOS 3.12 returns MULTIPOINT ZM EMPTY, but also accept POINT ZM EMPTY
41:         # to be future proof...
41: >       assert g2.ExportToIsoWkt() in ("MULTIPOINT ZM EMPTY", "POINT ZM EMPTY")
41: E       AssertionError: assert 'POINT EMPTY' in ('MULTIPOINT ZM EMPTY', 'POINT ZM EMPTY')

This passed with 3.13.0