Fix default values

app/Migrations/Schema/V110/Version110.php

@@ -45,6 +45,9 @@ class Version110 extends AbstractMigrationChamilo
         $this->addSql("CREATE TABLE IF NOT EXISTS c_student_publication_comment (iid INT NOT NULL PRIMARY KEY, id INT NULL, work_id INT NOT NULL, c_id INT NOT NULL, comment text, file VARCHAR(255), user_id int NOT NULL, sent_at datetime NOT NULL)");
         $this->addSql("CREATE TABLE IF NOT EXISTS c_attendance_calendar_rel_group (iid int NOT NULL auto_increment PRIMARY KEY, id INT, c_id INT NOT NULL, group_id INT NOT NULL, calendar_id INT NOT NULL)");
 
+        $this->addSql("ALTER TABLE session MODIFY COLUMN date_start date default NULL, MODIFY COLUMN date_end date default NULL");
+        $this->addSql("ALTER TABLE skill_rel_user MODIFY COLUMN acquired_skill_at datetime default NULL");
+
         $this->addSql("ALTER TABLE skill_rel_user ADD COLUMN course_id INT NOT NULL DEFAULT 0 AFTER id");
         $this->addSql("ALTER TABLE skill_rel_user ADD COLUMN session_id INT NOT NULL DEFAULT 0 AFTER course_id");
         $this->addSql("ALTER TABLE skill_rel_user ADD INDEX idx_select_cs (course_id, session_id)");
@@ -54,8 +57,7 @@ class Version110 extends AbstractMigrationChamilo
         if (!$session->hasColumn('description')) {
             $session->addColumn(
                 'description',
-                'text',
-                array('default' => 'NULL')
+                'text'
             );
         }
 
@@ -69,7 +71,7 @@ class Version110 extends AbstractMigrationChamilo
 
         $this->addSql("ALTER TABLE session ADD COLUMN duration int");
         $this->addSql("ALTER TABLE session_rel_user ADD COLUMN duration int");
-        $this->addSql("ALTER TABLE skill ADD COLUMN criteria text DEFAULT ''");
+        $this->addSql("ALTER TABLE skill ADD COLUMN criteria text");
         $this->addSql("ALTER TABLE gradebook_category ADD COLUMN generate_certificates TINYINT NOT NULL DEFAULT 0");
         $this->addSql("ALTER TABLE track_e_access ADD COLUMN c_id int NOT NULL");