Handle absence of rdef.coord_dimension in pg_geometry_sqltype

Fixes crash on python3 due to NoneType/int comparison.

site_cubicweb.py

@@ -27,7 +27,7 @@ def pg_geometry_sqltype(rdef):
     and coord_dimension.
     """
     target_geom_type = rdef.geom_type
-    if rdef.coord_dimension >= 3:  # XXX: handle 2D+M
+    if rdef.coord_dimension is not None and rdef.coord_dimension >= 3:  # XXX: handle 2D+M
         target_geom_type += 'Z'
     if rdef.coord_dimension == 4:
         target_geom_type += 'M'