Handle errors=None in _finalize_traitement

Sometimes the `errors` variable is `None` in `_finalize_traitement`.
It was not an issue when it was just used in the templates but now we
have to handle this in the view. Basically, we shall consider it is an
empty list when it occurs.

gestioncof/petits_cours_views.py

@@ -102,6 +102,7 @@ def _finalize_traitement(request, demande, proposals, proposed_for,
             'style="width:99%; height: 90px;">'
             '</textarea>'
     })
+    if errors is not None:
         for error in errors:
             messages.error(request, error)
     return render(request, "gestioncof/traitement_demande_petit_cours.html",