Fix encrypted video playback in Chrome-based browsers
For Chrome-based browsers, it seems we need to set some non-empty `src` URI for
the video element's play button to be enabled, so this crafts an empty `data`
URI and ensures playing is triggered once the real content has been fetched.
Fixes https://github.com/vector-im/element-web/issues/15694
Regressed by https://github.com/matrix-org/matrix-react-sdk/pull/5352