MDL-72309 course: make category a required field in forms.

Course category fields were changed en masse from simple select
elements to autocompletes in 93d72205, but weren't defined as
required fields. This could lead to exceptions and/or unexpected
behaviour if their values were cleared prior to form submission.

admin/tool/uploadcourse/classes/step2_form.php

@@ -84,6 +84,7 @@ class tool_uploadcourse_step2_form extends tool_uploadcourse_base_form {
 
         $displaylist = core_course_category::make_categories_list('moodle/course:create');
         $mform->addElement('autocomplete', 'defaults[category]', get_string('coursecategory'), $displaylist);
+        $mform->addRule('defaults[category]', null, 'required', null, 'client');
         $mform->addHelpButton('defaults[category]', 'coursecategory');
 
         $choices = array();