hswg: make AttrList return nil for missing attrs

And update copyright years.

LICENSE

@@ -1,4 +1,4 @@
-Copyright (c) 2018 - 2021, Přemysl Eric Janouch <p@janouch.name>
+Copyright (c) 2018 - 2022, Přemysl Eric Janouch <p@janouch.name>
 
 Permission to use, copy, modify, and/or distribute this software for any
 purpose with or without fee is hereby granted.

hswg/main.go

@@ -49,6 +49,10 @@ func (m *Metadata) Attr(name string) string {
 // AttrList is similar to Attr, but splits the result at commas,
 // and trims whitespace around array elements.
 func (m *Metadata) AttrList(name string) []string {
+	if !m.Attributes.Has(name) {
+		return nil
+	}
+
 	res := strings.Split(m.Attr(name), ",")
 	for i := range res {
 		res[i] = strings.TrimSpace(res[i])