空的 List<Channel> 为什么不能直接复制成 EnumSet？

import java.util.*;
public class EnumInputDemo {
    enum Channel { EMAIL, SMS, PUSH }
    static int checks;
    static void check(boolean ok) { if(!ok) throw new AssertionError(); checks++; }
    static List<Channel> decode(List<String> names) {
        List<Channel> result=new ArrayList<>();
        for(String name:names) result.add(Channel.valueOf(name));
        return result;
    }
    static EnumSet<Channel> selected(List<String> names) {
        EnumSet<Channel> result=EnumSet.noneOf(Channel.class);
        result.addAll(decode(names)); return result;
    }
    static boolean copyFails(Collection<Channel> input) {
        try { EnumSet.copyOf(input); return false; }
        catch(IllegalArgumentException expected) { return true; }
    }
    public static void main(String[] args) {
        List<Channel> decodedEmpty=decode(List.of());
        check(copyFails(decodedEmpty));
        EnumSet<Channel> empty=selected(List.of());
        check(empty.isEmpty()); check(EnumSet.copyOf(empty).isEmpty());
        Collection<Channel> sameObject=empty;
        check(EnumSet.copyOf(sameObject).isEmpty());
        System.out.println("empty_List_copy=IllegalArgumentException; empty_EnumSet_copy=ok; Collection_reference_to_EnumSet=ok");
        Set<Channel> wrapped=Collections.unmodifiableSet(empty);
        check(copyFails(wrapped));
        System.out.println("unmodifiable_empty_wrapper_copy=IllegalArgumentException");
        EnumSet<Channel> picked=selected(List.of("SMS","EMAIL","SMS"));
        check(picked.equals(EnumSet.of(Channel.EMAIL,Channel.SMS)));
        check(new ArrayList<>(picked).equals(List.of(Channel.EMAIL,Channel.SMS)));
        System.out.println("input=[SMS, EMAIL, SMS]; selected="+picked);
        EnumSet<Channel> remaining=EnumSet.complementOf(picked);
        check(remaining.equals(EnumSet.of(Channel.PUSH)));
        check(EnumSet.complementOf(empty).equals(EnumSet.allOf(Channel.class)));
        System.out.println("not_selected="+remaining+"; complement_of_empty="+EnumSet.complementOf(empty));
        boolean unknown=false,space=false;
        try { selected(List.of("FAX")); } catch(IllegalArgumentException e) { unknown=true; }
        try { selected(List.of(" SMS ")); } catch(IllegalArgumentException e) { space=true; }
        check(unknown); check(space);
        System.out.println("unknown_name=rejected; uncontracted_whitespace=rejected");
        System.out.println("PASS checks="+checks);
    }
}