fix: allowed ConcatenateJSON with empty input

pkg/utils/marshal.go

@@ -29,6 +29,14 @@ func ConcatenateJSON(first, second []byte) ([]byte, error) {
 	if !bytes.HasPrefix(second, []byte{'{'}) {
 		return nil, fmt.Errorf("jws: invalid JSON %s", second)
 	}
+	// check empty
+	if len(first) == 2 {
+		return second, nil
+	}
+	if len(second) == 2 {
+		return first, nil
+	}
+
 	first[len(first)-1] = ','
 	first = append(first, second[1:]...)
 	return first, nil