bug related to postgres - inserts into prefix_exercise_assessments were failing,

because generalcomment and teachercomment were not null and didn't have defaults.
for some reason mysql allows this but postgres complains. I've set empty string defaults and added into mysql for consistency.

mod/exercise/db/postgres7.php

@@ -20,6 +20,10 @@ function exercise_upgrade($oldversion) {
         table_column("exercise", "", "password", "VARCHAR", "32", "", "", "NOT NULL");
     }
 
+    if ($oldversion < 2004091000) {
+        table_column("exercise_assessments","generalcomment","generalcomment","text","","","","NOT NULL");
+        table_column("exercise_assessments","teachercomment","teachercomment","text","","","","NOT NULL");
+    }
 
     return true;
 }