Support zero args in variadic properties

convert.go

@@ -497,8 +497,12 @@ func (p prop) assign(dst S, args string) (S, error) {
 	vals = append(vals, reflect.ValueOf(dst))
 	pos := 0
 	input := []byte(args)
-	for _, arg := range p.args {
+	for i, arg := range p.args {
 		if pos >= len(input) {
+			if p.isVariadic && i == len(p.args)-1 {
+				// It's ok for a variadic arg list to have zero argument.
+				break
+			}
 			return dst, fmt.Errorf("missing value")
 		}
 		var err error

convert_test.go

@@ -75,6 +75,18 @@ border-bottom-size: 1;
 border-style: border("a","b","c","d","e","f","g","h");
 border-top: true;
 border-top-width: 1;`, ``},
+		{emptyStyle,
+			`border: border("a","b","c","d","e","f","g","h")`,
+			`border-bottom: true;
+border-bottom-size: 1;
+border-left: true;
+border-left-size: 1;
+border-right: true;
+border-right-size: 1;
+border-style: border("a","b","c","d","e","f","g","h");
+border-top: true;
+border-top-width: 1;`,
+			``},
 		{emptyStyle,
 			`border: border("a","b","c","d","e","f","g","h") true xx`,
 			``,