MDL-58030 gradingform_guide: Init $commentchooser before use

The initialisation of $commentschooser was moved into an IF condition,
whilst still using it outside of that IF conditions scope. As such, if
there are no comments to choose from, the variable is never init'd but
it still goes to use it, emitting a PHP notice.

grade/grading/form/guide/renderer.php

@@ -236,6 +236,7 @@ public function criterion_template($mode, $options, $elementname = '{NAME}', $cr
             $input = html_writer::tag('textarea', s($currentremark), $remarkparams);
 
             // Show the frequently-used comments chooser only if there are defined entries.
+            $commentchooser = '';
             if (!empty($comments)) {
                 // Frequently used comments chooser.
                 $chooserbuttonid = 'criteria-' . $criterion['id'] . '-commentchooser';