MDL-78775 core_grades: Fix saving on grader report page

When we have a pagination on grader page and search for user
that is not on first page, saving was throwing an error
since page didn't have enough data

grade/lib.php

@@ -1051,6 +1051,12 @@ class grade_plugin_return {
      * @var int
      */
     public $page;
+    /**
+     * Search string
+     *
+     * @var string
+     */
+    public $search;
 
     /**
      * Constructor
@@ -1064,6 +1070,7 @@ public function __construct($params = []) {
         $this->userid   = optional_param('gpr_userid', null, PARAM_INT);
         $this->groupid  = optional_param('gpr_groupid', null, PARAM_INT);
         $this->page     = optional_param('gpr_page', null, PARAM_INT);
+        $this->search   = optional_param('gpr_search', '', PARAM_NOTAGS);
 
         foreach ($params as $key => $value) {
             if (property_exists($this, $key)) {
@@ -1217,6 +1224,12 @@ public function get_form_fields() {
         if (!empty($this->page)) {
             $result .= '<input type="hidden" name="gpr_page" value="'.$this->page.'" />';
         }
+
+        if (!empty($this->search)) {
+            $result .= html_writer::empty_tag('input',
+                ['type' => 'hidden', 'name' => 'gpr_search', 'value' => $this->search]);
+        }
+
         return $result;
     }