empty wkt multipolygon does not roundtrip

Nobody has claimed this yet.

If you start with
```js
const wkt = 'MULTIPOLYGON EMPTY'
```
and then try to roundtrip it using
```js
const newWkt = stringify(parse(wkt))
```
you end up with `MULTIPOLYGON ()`. which in turn parses to `null` (which cannot be stringified anymore).

So in summary:
- `MULTIPOLYGON EMPTY` parses just fine, to a geometry with empty coordinates array
- a multipolygon geometry with empty coordinates array stringifies to `MULTIPOLYGON ()` instead of `MULTIPOLYGON EMPTY`, and is treated as invalid syntax when re-parsing it, resulting in a `null` parsing result

Start by running the JavaScript parse/stringify example from the issue and inspect how empty multipolygon coordinates are handled by both entry points. Done means `MULTIPOLYGON EMPTY` parses and stringifies back to valid WKT that parses again without producing `null`.

Written by the indexing model from the issue text.