YouTube: Treat channels without supported tabs as empty channels

[InfinityLoop1308]

Description

Some valid YouTube channels do not have any supported content tabs.

YoutubeChannelExtractor#getVideoTab() currently throws:

ContentNotSupportedException: This channel has no supported tabs

Such channels are valid and their metadata can still be extracted. Having no supported tabs should mean that the channel contains no extractable items, rather than that the channel itself is unsupported.

Expected behavior

For a channel without supported tabs:

• getInitialPage() returns an empty page
• getTabs() returns an empty list
• ChannelInfo does not contain a ContentNotSupportedException