Use biomegen.make_void_maps if present.

(latest version of biomegen)

mapgen.lua

@@ -134,6 +134,9 @@ function mapgen_rivers.make_chunk(minp, maxp, seed)
 
 			minetest.log("verbose", "[mapgen_rivers] Skipping empty chunk (fully above ground level)")
 			minetest.log("verbose", ("[mapgen_rivers] Done in %5.3f s"):format(t))
+			if use_biomegen_mod and biomegen.make_void_maps then
+				biomegen.make_void_maps(minp, maxp)
+			end
 			return
 		end
 	end