stream.dash_manifest: Fixed bug for dynamic playlists when parent.id is None
stream.dash_manifest: Fixed bug for dynamic playlists when parent.id is None

if `self.parent.id` is None, use `self.parent.mimeType` instead.

This will create two timelines for video and audio,
instead of one for both which won't work.

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Test URLS:

https://www.france.tv/france-5/direct.html

http://livesim.dashif.org/livesim/segtimeline_1/testpic_2s/Manifest.mpd
from https://github.com/Dash-Industry-Forum/dash-live-source-simulator/wiki/Test-URLs

melmorabity/streamlink,chhe/streamlink,gravyboat/streamlink,beardypig/streamlink,chhe/streamlink,bastimeyer/streamlink,wlerin/streamlink,back-to/streamlink,streamlink/streamlink,gravyboat/streamlink,wlerin/streamlink,streamlink/streamlink,beardypig/streamlink,melmorabity/streamlink,bastimeyer/streamlink,back-to/streamlink