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/outputcomponents.php

@@ -2640,7 +2640,7 @@ class html_table {
      * $row2->cells = array($cell2, $cell3);
      * $t->data = array($row1, $row2);
      */
-    public $data;
+    public $data = [];
 
     /**
      * @deprecated since Moodle 2.0. Styling should be in the CSS.