db: Make 'text' field in 'comments' table NOT NULL and handling data …

…migration

This update introduces a schema migration to version 4 for the database, focusing on enhancing the 'comments' table. This ensures that the 'text' field in the 'comments' table will always have a value, which improves data consistency and integrity.

See:
- #979
- #994

isso/tests/test_db.py

@@ -267,7 +267,7 @@ def test_comment_text_not_null_migration_with_rollback_after_error(self):
         })
 
         try:
-            db = SQLite3(self.path, conf)
+            SQLite3(self.path, conf)
         except RuntimeError as e:
             self.assertEqual(str(e), "Migrating DB version 4 to 5 failed: no such column: notification")