fix for WP 3.5 compatibility

fix for a warning message introduced with WP 3.5, see http://make.wordpress.org/core/2012/12/12/php-warning-missing-argument-2-for-wpdb-prepare/ for more info.

this fix doesn't change current GTM security status, the prepare statement were correctly and already used almost everywhere with parameters, here I'm adding an empty string parameter to those that were missing it.

The code needs review to see if some more sql queries need refactorings.

Quickly find where an empty parameter is passed with the following regex

\$wpdb->prepare\(.*,''\s*+\)