fix if videoEl.dataset[c] is undefined , JSON.parse will crash

src/video/video.js

@@ -44,7 +44,7 @@ export default (
                 PROPERTIES.reduce((p, c) => {
                     p[c] = this.videoEl && this.videoEl[c];
                     if (c == 'playbackrates' && this.videoEl) {
-                        if (this.videoEl.dataset) {
+                        if (this.videoEl.dataset && this.videoEl.dataset[c]) {
                             p[c] = JSON.parse(this.videoEl.dataset[c]);
                         } else {
                             p[c] = JSON.parse(this.videoEl.getAttribute("data-" + c));