[youtube] Fix empty description extraction

[dpeukert]

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Description of your pull request and other information

In its current state, the YouTube description extraction logic only checks if shortDescription from video_details is truthy, which it is not if it's an empty string when there is no description. As a result of that, it uses the description meta tag, which has a default value of Enjoy the videos and music you love, upload original content, and share it all with friends, family, and the world on YouTube.. This PR changes this behaviour to only use the meta tag if shortDescription is None (= doesn't exist) and adds a test for it.

[0x9fff00]

Fixes #26006