Fields in `media.metadata` (artist, title etc) never get cleared in `MediaStatus` and can report wrong/stale info.

[mikerofone]

Summary

Status updates don't always have all media fields popuplated. The current MediaStatus.update() method only updates media_data-related fields with non-empty values but never clears them. This results in stale title/artist/album being reported, for example, when switching from "real" YouTube Music songs (media.metadata populated) to YouTube videos that can also be accessed through YouTube Music (media.metadata unpoulated).

Reproduction steps

1. From phone or desktop, start playing YouTube music on a Chromecast on a "proper", label-provided YouTube Music song, e.g. https://music.youtube.com/watch?v=QAvV1iKljmI
2. Register a MediaStatusListener to the Chromecast.
3. Observe that MediaStatus contains the correct title ("Tuoppein'nostelulaulu" for the given example)
4. Switch the song to a song that's a regular YouTube video uploaded by a random channel, e.g. https://music.youtube.com/watch?v=IrSRcotvzhk.

Expected behavior

The MediaStatusListener from step 2 eventually receives an update that either has the correct title for the song chosen in step 4 ("BONK! - Für immer") or an empty title.

Actual behavior

All following updates still contain the title of the song chosen in step 1 ("Tuoppein'nostelulaulu") even though another song is playing.

Suggested Change

If a status update is received that sets media.metadata, all metadata-related fields in the MediaStatus object should be cleared if not set in the latest update, instead of reusing their old values.

Possible caveats

The suggested change fixes the staleness in my scenario, but I have no idea whether the carrying over of old values was a deliberate decision to fix issues with other scenarios. Wanted to discuss the issue first before sending a PR.

Example status updates

Output of pprint.pprint(data) at the top of MediaStatus.update().

For proper YTM song

{'status': [{'currentTime': 0.857,
             'customData': {'playerState': 1},
             'media': {'contentId': 'QAvV1iKljmI',
                       'contentType': 'x-youtube/video',
                       'customData': {'currentIndex': 0,
                                      'listId': 'RQ7rX-LBVjUAyfQO5oKou_yN6ZT9I'},
                       'duration': 395,
                       'metadata': {'albumName': 'Betrayal, Justice, Revenge',
                                    'artist': 'Kivimetsän Druidi',
                                    'images': [{'height': 544,
                                                'url': 'https://lh3.googleusercontent.com/01qD63QrFeoSqw7UhW97SMhcqQrFy1k8haonCt2LKwY56TVv7IdKE8BLHbClH_dB7BQR9U6gs7KQwGE=w544-h544-l90-rj',
                                                'width': 544},
                                               {'height': 302,
                                                'url': 'https://lh3.googleusercontent.com/01qD63QrFeoSqw7UhW97SMhcqQrFy1k8haonCt2LKwY56TVv7IdKE8BLHbClH_dB7BQR9U6gs7KQwGE=w302-h302-l90-rj',
                                                'width': 302},
                                               {'height': 180,
                                                'url': 'https://lh3.googleusercontent.com/01qD63QrFeoSqw7UhW97SMhcqQrFy1k8haonCt2LKwY56TVv7IdKE8BLHbClH_dB7BQR9U6gs7KQwGE=w180-h180-l90-rj',
                                                'width': 180},
                                               {'height': 120,
                                                'url': 'https://lh3.googleusercontent.com/01qD63QrFeoSqw7UhW97SMhcqQrFy1k8haonCt2LKwY56TVv7IdKE8BLHbClH_dB7BQR9U6gs7KQwGE=w120-h120-l90-rj',
                                                'width': 120},
                                               {'height': 60,
                                                'url': 'https://lh3.googleusercontent.com/01qD63QrFeoSqw7UhW97SMhcqQrFy1k8haonCt2LKwY56TVv7IdKE8BLHbClH_dB7BQR9U6gs7KQwGE=w60-h60-l90-rj',
                                                'width': 60}],
                                    'metadataType': 3,
                                    'title': "Tuoppein'nostelulaulu"},
                       'streamType': 'BUFFERED'},
             'mediaSessionId': 1794657190,
             'playbackRate': 1,
             'playerState': 'PLAYING',
             'supportedMediaCommands': 311363,
             'volume': {'level': 1, 'muted': False}}],
 'type': 'MEDIA_STATUS'}

For YouTube video

{'status': [{'currentTime': 1.029,
             'customData': {'playerState': 1},
             'media': {'contentId': 'IrSRcotvzhk',
                       'contentType': 'x-youtube/video',
                       'customData': {'currentIndex': 0,
                                      'listId': 'RQ7rX-LBVjUAyfQO5oKou_yN6ZT9I'},
                       'duration': 175,
                       'streamType': 'BUFFERED'},
             'mediaSessionId': 1794657190,
             'playbackRate': 1,
             'playerState': 'PLAYING',
             'supportedMediaCommands': 311363,
             'volume': {'level': 1, 'muted': False}}],
 'type': 'MEDIA_STATUS'}