Remove uninitialized var

[nigrosimone]

Into Cdn_Plugin.php there is a uninitialized variable $empty, see:

( $this->can_cdn() && $this->can_cdn2( $empty ) ) ) {

this cause an error:

Notice: Undefined variable: $empty in Cdn_Plugin.php on line 777

the fix is replace $empty with an empty string:

( $this->can_cdn() && $this->can_cdn2( '' ) ) ) {

This PR fix the issue #470

Thanks to @Furniel for the fix in #477