Fix inconsistencies in database migrations

In 14442c0, `updatetime` field was
introduced to the `items` table. Unfortunately, unlike the
initialization, the migration did not flag the field `NOT NULL`.
Additionally, the `lastseen` column in MySQL database was added as
`NULL`able in 83794f4. These
inconsistencies can lead to incorrect assumptions and cause bugs like #954.

This patch adds the `NOT NULL` constraint for `updatetime` and `lastseen`
fields, making the database consistent again. SQLite consistency was
already rectified in 83794f4 by
re-creating the table.

daos/mysql/Database.php

@@ -235,6 +235,14 @@ public function __construct() {
                         'INSERT INTO ' . \F3::get('db_prefix') . 'version (version) VALUES (11)'
                     ]);
                 }
+                if (strnatcmp($version, '12') < 0) {
+                    \F3::get('db')->exec([
+                        'UPDATE ' . \F3::get('db_prefix') . 'items SET updatetime = datetime WHERE updatetime IS NULL',
+                        'ALTER TABLE ' . \F3::get('db_prefix') . 'items MODIFY updatetime DATETIME NOT NULL',
+                        'ALTER TABLE ' . \F3::get('db_prefix') . 'items MODIFY lastseen DATETIME NOT NULL',
+                        'INSERT INTO ' . \F3::get('db_prefix') . 'version (version) VALUES (12)'
+                    ]);
+                }
             }
 
             // just initialize once

daos/pgsql/Database.php

@@ -220,6 +220,13 @@ public function __construct() {
                         'INSERT INTO version (version) VALUES (11);'
                     ]);
                 }
+                if (strnatcmp($version, '12') < 0) {
+                    \F3::get('db')->exec([
+                        'UPDATE items SET updatetime = datetime WHERE updatetime IS NULL',
+                        'ALTER TABLE items ALTER COLUMN updatetime SET NOT NULL',
+                        'INSERT INTO version (version) VALUES (12)'
+                    ]);
+                }
             }
 
             // just initialize once