Avoid a PHP notice in wp_new_comment() if user ID is not passed.

see #23231.
Built from https://develop.svn.wordpress.org/trunk@28922


git-svn-id: http://core.svn.wordpress.org/trunk@28721 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Pass $url parameter to wp_embed_defaults() and 'embed_defaults' filter.

props wpsmith, sabreuse, jacobdubail.
fixes #20151.
Built from https://develop.svn.wordpress.org/trunk@28923


git-svn-id: http://core.svn.wordpress.org/trunk@28722 1a063a9b-81f0-0310-95a4-ce76da25c4cd