MDL-57680 form autocomplete: let singleselects be cleared.

This was already working after ajax had refreshed the choices, it was just
broken on first page load.

lib/amd/src/form-autocomplete.js

@@ -1026,6 +1026,12 @@ function($, log, str, templates, notification, LoadingIcon) {
             uniqueId++;
 
             options.multiple = originalSelect.attr('multiple');
+            if (!options.multiple) {
+                // If this is a single select then there is no way to de-select the current value -
+                // unless we add a bogus blank option to be selected when nothing else is.
+                // This matches similar code in updateAjax above.
+                originalSelect.prepend('<option>');
+            }
 
             if (typeof closeSuggestionsOnSelect !== "undefined") {
                 options.closeSuggestionsOnSelect = closeSuggestionsOnSelect;

lib/form/course.php

@@ -124,7 +124,7 @@ class MoodleQuickForm_course extends MoodleQuickForm_autocomplete {
         $coursestofetch = array();
 
         foreach ($values as $onevalue) {
-            if ((!$this->optionExists($onevalue)) &&
+            if ($onevalue && !$this->optionExists($onevalue) &&
                     ($onevalue !== '_qf__force_multiselect_submission')) {
                 array_push($coursestofetch, $onevalue);
             }