Goでnilなsliceやmapを返すと空のsliceやmapになる

package main

import "fmt"

func emptyMap() map[string]string {
    return nil
}

func emptySlice() []string {
    return nil
}

func main() {
    emptySlice := emptySlice()
    fmt.Printf("slice len = %d\n", len(emptySlice))

    emptyMap := emptyMap()
    fmt.Printf("map len = %d\n", len(emptyMap))
}