TransformXY doesn't preserve coordinates type for some empty collections

[peterstace]

The following program exhibits the buggy behaviour:

package main

import (
    "fmt"

    "github.com/peterstace/simplefeatures/geom"
)

func main() {
    tx("POLYGON Z EMPTY")
    tx("MULTILINESTRING Z EMPTY")
    tx("MULTIPOINT Z EMPTY")
    tx("GEOMETRYCOLLECTION Z EMPTY")
}

func must(err error) {
    if err != nil {
        panic(err)
    }
}

func tx(wkt string) {
    g, err := geom.UnmarshalWKT(wkt)
    must(err)
    t, err := g.TransformXY(func(in geom.XY) geom.XY { return in })
    must(err)
    fmt.Println(t.AsText())
}

Outputs:

POLYGON Z EMPTY
MULTILINESTRING EMPTY
MULTIPOINT EMPTY
GEOMETRYCOLLECTION Z EMPTY

The correct output should be:

POLYGON Z EMPTY
MULTILINESTRING Z EMPTY
MULTIPOINT Z EMPTY
GEOMETRYCOLLECTION Z EMPTY