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.


Built from https://develop.svn.wordpress.org/trunk@28559


git-svn-id: http://core.svn.wordpress.org/trunk@28385 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Truly check for `! empty()` instead of falsey `''` when determining whether to add `LIMIT` clause to SQL in `wp_get_archives()`.

Props jjeaton for the initial patch.
Fixes #27834.

Built from https://develop.svn.wordpress.org/trunk@28560


git-svn-id: http://core.svn.wordpress.org/trunk@28386 1a063a9b-81f0-0310-95a4-ce76da25c4cd