CArrayDataProvider cast sort field to strlower issue

[sveneld]

Deprecated: mb_strtolower(): Passing null to parameter #1 ($string) of type string is deprecated in vendor/yiisoft/yii/framework/web/CArrayDataProvider.php on line 179

protected function getSortingFieldValue($data, $fields)
	{
		if(is_object($data))
		{
			foreach($fields as $field)
				$data=isset($data->$field) ? $data->$field : null;
		}
		else
		{
			foreach($fields as $field)
				$data=isset($data[$field]) ? $data[$field] : null;
		}
		return $this->caseSensitiveSort ? $data : mb_strtolower($data,Yii::app()->charset);
	}

What steps will reproduce the problem?

If your dataprovider have a field with null value and caseSensitiveSort = false you will have a notice due to the casting $data to string

What is the expected result?

No deprecation notice

What do you get instead?

deprecation notice

Additional info

Q	A
Yii version	1.1.29
PHP version	8.1
Operating system