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.



git-svn-id: https://develop.svn.wordpress.org/trunk@28559 602fd350-edb4-49c9-b593-d223f7449a82