MDL-52423 tool_lp: Allow null related objects in exporters

Note that the related objects MUST always ALL be passed to the
constructor when instantiating objets, even if they are null.

Related objects were introduced to improve performance and their
requirement is one way to ensure that developers don't forget them.

admin/tool/lp/classes/external/exporter.php

@@ -63,13 +63,26 @@ abstract class exporter {
         // Cache the valid related objects.
         foreach (static::define_related() as $key => $classname) {
             $isarray = false;
+            $nullallowed = false;
+
+            // Allow ? to mean null is allowed.
+            if (substr($classname, -1) === '?') {
+                $classname = substr($classname, 0, -1);
+                $nullallowed = true;
+            }
+
             // Allow [] to mean an array of values.
             if (substr($classname, -2) === '[]') {
                 $classname = substr($classname, 0, -2);
                 $isarray = true;
             }
+
             $missingdataerr = 'Exporter class is missing required related data: (' . get_called_class() . ') ';
-            if ($isarray) {
+
+            if ($nullallowed && array_key_exists($key, $related) && $related[$key] === null) {
+                $this->related[$key] = $related[$key];
+
+            } else if ($isarray) {
                 if (array_key_exists($key, $related) && is_array($related[$key])) {
                     foreach ($related[$key] as $index => $value) {
                         if (!$value instanceof $classname) {
@@ -80,8 +93,9 @@ abstract class exporter {
                 } else {
                     throw new coding_exception($missingdataerr . $key . ' => ' . $classname . '[]');
                 }
+
             } else {
-                if ((array_key_exists($key, $related) && ($related[$key] instanceof $classname))) {
+                if (array_key_exists($key, $related) && $related[$key] instanceof $classname) {
                     $this->related[$key] = $related[$key];
                 } else {
                     throw new coding_exception($missingdataerr . $key . ' => ' . $classname);
@@ -272,7 +286,9 @@ abstract class exporter {
      *
      * Only objects listed here can be cached in this object.
      *
-     * The class name can be suffixed with [] to indicate an array of values.
+     * The class name can be suffixed:
+     * - with [] to indicate an array of values.
+     * - with ? to indicate that 'null' is allowed.
      *
      * @return array of 'propertyname' => array('type' => classname, 'required' => true)
      */