In moodle 1.8 and 1.9 (not tested on 1.7) when student clicks on submit button and response is empty, there ought to be displayed the error message: "No answer given. Please go back and submit an answer."
In moodle 1.8.2, this bug can be fixed by moving line 651
$feedback = get_string('noanswer', 'lesson');
up to just after line 526, which should now be: