Default configuration value for serialized_array settings is ignored

After some tests I can confirm this is a bug introduced by Magento 1.9.3.0 (see here: https://github.com/OpenMage/magento-mirror/commit/d48bebc211cc216aaf78bdf25d7f0b0143d6333b#diff-139e884940505308d9c796f5e3a78865 )

Side note: this also affects SUPEE-8788

As a temporary fix, here is what I suggest: copy app/code/core/Mage/Adminhtml/Model/System/Config/Backend/Serialized.php to app/code/local/Mage/Adminhtml/Model/System/Config/Backend/Serialized.php and modify the _afterLoad() method like this:

protected function _afterLoad()
{
    if (!is_array($this->getValue())) {
        $serializedValue = $this->getValue();
        $unserializedValue = false;
        if (!empty($serializedValue)) {
            try {
                $unserializedValue = Mage::helper('core/unserializeArray')
                    ->unserialize((string)$serializedValue);
            } catch (Exception $e) {
                Mage::logException($e);
            }
        }
        $this->setValue($unserializedValue);
    }
}