MDL-60281 general: various strict corrections for PHP7.2

- count() can only be called on arrays or Countable, it can not be called on null
- recordset is neither so iterator_count() should be used
- instanceof or get_class() can not be applied to non-objects
- class methods must have the same arguments as methods in parent class

lib/navigationlib.php

@@ -843,7 +843,7 @@ class navigation_node implements renderable {
  * @copyright 2010 Sam Hemelryk
  * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
-class navigation_node_collection implements IteratorAggregate {
+class navigation_node_collection implements IteratorAggregate, Countable {
     /**
      * A multidimensional array to where the first key is the type and the second
      * key is the nodes key.