When live-updating site title in toolbar, fall back to the site url if the title is empty.

props avryl.
see #28682.
Built from https://develop.svn.wordpress.org/trunk@30597


git-svn-id: http://core.svn.wordpress.org/trunk@30587 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Consistently handle an empty site title in Site and My Sites toolbar menus.

see #28682.
Built from https://develop.svn.wordpress.org/trunk@30598


git-svn-id: http://core.svn.wordpress.org/trunk@30588 1a063a9b-81f0-0310-95a4-ce76da25c4cd