MDL-59173 tablelib.php: changed default of $params in set_sql to array()

Default of $params = null leads to a problem in query_db().
There this value is used in array_merge, which can not handle null
values. If null is passed for one of the params the outcome will be null
independent of the value of the second param!

lib/tablelib.php

@@ -1545,7 +1545,7 @@ class table_sql extends flexible_table {
      * Of course you can use sub-queries, JOINS etc. by putting them in the
      * appropriate clause of the query.
      */
-    function set_sql($fields, $from, $where, array $params = NULL) {
+    function set_sql($fields, $from, $where, array $params = array()) {
         $this->sql = new stdClass();
         $this->sql->fields = $fields;
         $this->sql->from = $from;