TinyMCE: don't remove empty <i>, <b>, <em> and <strong> matching the behavior...

TinyMCE: don't remove empty <i>, <b>, <em> and <strong> matching the behavior in WordPress 3.4/TinyMCE 3.4.9, see #22477 for trunk

git-svn-id: https://core.svn.wordpress.org/trunk@23222 1a063a9b-81f0-0310-95a4-ce76da25c4cd