Prevent ST_Reverse from converting MP -> GC

Before we had:
```mysql
SELECT ST_Reverse('MULTIPOINT((4 4), (1 1), (1 0), (0 3))');
-- Answer: GEOMETRYCOLLECTION(POINT(4 4), POINT(1 1),
--                            POINT(1 0), POINT(0 3))
```

Now we have
```mysql
SELECT ST_Reverse('MULTIPOINT((4 4), (1 1), (1 0), (0 3))');
-- Answer: MULTIPOINT((0 3), (1 0), (1 1), (4 4))
```

Also return `NULL` for `NULL` input geometries.