Change seen_by in msgdata_template to not be null

The sender has always seen their message, haven't they?

src/sqlite/msgdata_template.sql

@@ -7,6 +7,6 @@ CREATE TABLE msgdata_{chat_id} (
 	msg_type INTEGER NOT NULL,
 	content TEXT,
 	hide_for TEXT,
-	seen_by TEXT,
+	seen_by TEXT NOT NULL,
 	CONSTRAINT PK_msgdata_{chat_id} PRIMARY KEY (msg_id, timestamp)
 );