Don't die in my-sites.php if the user has no sites. Intead, display message...

Don't die in my-sites.php if the user has no sites.  Intead, display message within admin chrome. Props trepmal. fixes #18935

git-svn-id: https://core.svn.wordpress.org/trunk@19323 1a063a9b-81f0-0310-95a4-ce76da25c4cd