Description
When editing an article (or any record that uses Joomla\CMS\Table\Table::checkIn()), clicking Close triggers:

Column 'checked_out' cannot be null

Stack trace excerpt

Joomla\Database\Exception\ExecutionFailureException: Column 'checked_out' cannot be null
#2 /libraries/src/Table/Table.php(1238)
#3 /libraries/src/MVC/Model/FormModel.php(111)
...

Cause
Table::checkIn() builds this SQL:

$nullDate = $this->_supportNullValue ? 'NULL' : $this->_db->quote($this->_db->getNullDate());
$nullID   = $this->_supportNullValue ? 'NULL' : '0';

Since _supportNull Value is true in 5.3.x, Joomla issues:

UPDATE #__content
SET checked_out = NULL,
    checked_out_time = NULL

However, in the core schema for #__content, both checked_out and checked_out_time are declared NOT NULL DEFAULT 0/0000-00-00 00:00:00,
so strict-mode MySQL rejects the query.

Steps to Reproduce

1. Use Joomla 5.3.4 with a default schema on MySQL 8.0 (strict mode on)

2. Edit any article

3. Click Close

Expected result

• Article closes normally.

Actual result

• Error 1048 – “Column 'checked_out' cannot be null”.

System information (as much as possible)

Environment

• Joomla 5.3.4 Stable
• PHP 8.3 (FPM / CLI)
• MySQL 8.0 / MariaDB 10.11 (STRICT TRANS TABLES enabled)

Suggested fix

In /libraries/src/Table/Table.php → checkIn(), always use non-NULL safe values:

$nullDate = $this->_db->quote($this->_db->getNullDate());
$nullID   = '0';

or guard it conditionally against field definitions.

Workaround

Patch that line manually or override via a small system plugin.