Handle annotations with "never" as timestamp

For instance, this video:
https://www.youtube.com/watch?v=Vmql3OZ7Hi0
Handle annotations without text

For instance <TEXT /> (empty tag) is present in this video:
https://www.youtube.com/annotations_invideo?video_id=Z0S_oYsmcMU
Handle annotations with singleton movingRegions

For instance, the following video has a single position:
https://www.youtube.com/annotations_invideo?video_id=M2cgZGv8LOQ
Handle annotations of unknown or private videos

Since there is no API endpoint, retrieving annotations of unknown
videos or of private videos (to which YouTube responds with 403)
should not raise an error, but simply not return any annotation.
Merge branch 'fix-annotations'