Correction prompt : add error : "empty-answer".

correction.py

@@ -53,7 +53,7 @@ You also need to give feedback to the student, in french :
  - possibly, what he should have done instead.
 Your feedback may contain LaTeX fragments written like `$a^2 + b^2 = c^2$`.
 
-If your score is note 4, you should always provide some feedback
+If your score is not 4, you should always provide some feedback
 explaining what's missing.
 
 For each piece of feedback, if it is related to a specific part of the
@@ -62,7 +62,9 @@ specific part of the answer. This `box_2d` should be in the form
 [ymin, xmin, ymax, xmax] normalized to 0-1000. If you do not provide
 one, set `box_2d` to `null`.
 
-If the answer is correct, there is no need to provide feedback.
+If the answer is correct, there is no need to provide feedback. You do
+not have to give positive feedback, but if you do, do not provide a
+`box_2d` for it.
 
 For example, if the student says a function is continuous when it
 isn't, provide the coordinates where the word «continuous» is. If a
@@ -81,6 +83,9 @@ In some case, you may find that either
  - You can find an answer to another question of the exercice (taking
    more than a couple of lines). Score the question you are supposed
    to score, but set `error` to \"additional-answer\".
+ - The answer to the question is empty, or the student has only
+   rewritten the statement of the question. In this case, set `error`
+   to \"empty-answer\" and do not provide any kind of feedback.
 If there's no error, set `error` to `\"\"`.
 
 You will answer using json describing a list of dictionary with a key