MDL-64151 form: autocomplete element remove last choice

This is the same situation as was in MDL-56864 but now in 'course', 'autocomplete' and 'select' elements
- if the element allows multiple selection it is impossible to remove the last element

lib/form/select.php

@@ -166,8 +166,9 @@ class MoodleQuickForm_select extends HTML_QuickForm_select implements templatabl
     */
     function exportValue(&$submitValues, $assoc = false)
     {
+        $emptyvalue = $this->getMultiple() ? [] : '';
         if (empty($this->_options)) {
-            return $this->_prepareValue(null, $assoc);
+            return $this->_prepareValue($emptyvalue, $assoc);
         }
 
         $value = $this->_findValue($submitValues);
@@ -187,7 +188,7 @@ class MoodleQuickForm_select extends HTML_QuickForm_select implements templatabl
         }
 
         if (empty($cleaned)) {
-            return $this->_prepareValue(null, $assoc);
+            return $this->_prepareValue($emptyvalue, $assoc);
         }
         if ($this->getMultiple()) {
             return $this->_prepareValue($cleaned, $assoc);
@@ -223,6 +224,7 @@ class MoodleQuickForm_select extends HTML_QuickForm_select implements templatabl
             $options[] = $o;
         }
         $context['options'] = $options;
+        $context['nameraw'] = $this->getName();
 
         return $context;
     }