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

props avryl.
see #28682.

git-svn-id: https://develop.svn.wordpress.org/trunk@30597 602fd350-edb4-49c9-b593-d223f7449a82
Consistently handle an empty site title in Site and My Sites toolbar menus.

see #28682.

git-svn-id: https://develop.svn.wordpress.org/trunk@30598 602fd350-edb4-49c9-b593-d223f7449a82