fix test error

packages/artplayer/src/player/flipMix.js

@@ -11,7 +11,8 @@ export default function flipMix(art) {
         get() {
             return $player.dataset.flip || 'normal';
         },
-        set(flip = 'normal') {
+        set(flip) {
+            if (!flip) flip = 'normal';
             if (flip === 'normal') {
                 delete $player.dataset.flip;
             } else {