MDL-34793 mod_assign Fix rendering error that causes student view to …

…break in some cases.

When feedback is switched on for a assignment, and the teacher enters a grade through the gradebook, the student
view would become broken, throwing a coding error. Added check that $status->grade is not empty before passing
to assign_feedback_XXX::is_empty(), as that function expects an object.

mod/assign/renderer.php

@@ -321,7 +321,7 @@ public function render_assign_feedback_status(assign_feedback_status $status) {
         }
 
         foreach ($status->feedbackplugins as $plugin) {
-            if ($plugin->is_enabled() && $plugin->is_visible() && !$plugin->is_empty($status->grade)) {
+            if ($plugin->is_enabled() && $plugin->is_visible() && !empty($status->grade) && !$plugin->is_empty($status->grade)) {
                 $row = new html_table_row();
                 $cell1 = new html_table_cell($plugin->get_name());
                 $pluginfeedback = new assign_feedback_plugin_feedback($plugin, $status->grade, assign_feedback_plugin_feedback::SUMMARY, $status->coursemoduleid, $status->returnaction, $status->returnparams);