Clean code and prepare a more powerfull phpunit check for sql forging.

htdocs/core/lib/functions.lib.php

@@ -12697,6 +12697,9 @@ function jsonOrUnserialize($stringtodecode)
  */
 function forgeSQLFromUniversalSearchCriteria($filter, &$errorstr = '', $noand = 0, $nopar = 0, $noerror = 0)
 {
+	if ($filter === '') {
+		return '';
+	}
 	if (!preg_match('/^\(.*\)$/', $filter)) {    // If $filter does not start and end with ()
 		$filter = '(' . $filter . ')';
 	}