Alembic: Export mesh as mesh, even when it has no vertices.

This makes it possible to have an animated / procedurally generated mesh
that starts empty and obtains data in later frames.

Fixes the export of an empty mesh with an Ocean Modifier, as described in
issue T51351.

source/blender/alembic/intern/abc_exporter.cc

@@ -580,7 +580,7 @@ void AbcExporter::createShapeWriter(Object *ob, Object *dupliObParent)
 		{
 			Mesh *me = static_cast<Mesh *>(ob->data);
 
-			if (!me || me->totvert == 0) {
+			if (!me) {
 				return;
 			}