prevent warning in correction view for tb exercise

views/exercises/tb_exercise/correct.php

@@ -21,7 +21,7 @@
         </ul>
 
         <div id="commented-<?= $exercise->id ?>">
-            <? if ($edit_solution): ?>
+            <? if ($!empty(edit_solution)): ?>
                 <? if ($exercise->getLayout() === 'markup'): ?>
                     <? $answer = $response[0] ?>
                 <? elseif ($exercise->getLayout() === 'code'): ?>