[DigitallySpeaking] Support uncommon DigitallySpeaking case

[zorbathut]

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• At least skimmed through adding new extractor tutorial and youtube-dl coding conventions sections
• Searched the bugtracker for similar pull requests
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What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

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Description of your pull request and other information

Some rare GDC Vault videos, hosted by DigitallySpeaking, have no speakerVideo element but store the entire video within slideVideo (https://gdcvault.com/play/1016624, https://gdcvault.com/play/1016625, more). Unfortunately, the DigitallySpeaking extractor considers that a fatal error and refuses to download them.

This change accepts videos without either a speakerVideo or a slideVideo element.

[dstftw]

Add a test.

[zorbathut]

Moved to #28970 .