Read the `src` attribute for the `[embed]` shortcode if the shortcode's body is empty.

These work:
`[embed src="https://www.youtube.com/watch?v=ZqD-LPOWIT0"]`
`[embed src="https://www.youtube.com/watch?v=ZqD-LPOWIT0"][/embed]`

If you do:
`[embed src="https://www.youtube.com/watch?v=ZqD-LPOWIT0"]http://any-other-url.com[/embed]`

... "http://any-other-url.com" will be the URL that is parsed.

Props kovshenin, aaroncampbell for making initial patches.
Fixes #24456.


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