Students can send an assignment without choosing any file.

File: mod/assignment/type/uploadsingle/upload_form.php

Fix :

//Below this line :
$mform->addElement('filepicker', 'assignment_file', get_string('uploadafile'), null, $instance['options']);

//Add this line
$mform->addRule('assignment_file', get_string('uploadnofilefound'), 'required', null, 'client');