The deprecation warning suggests when parsing a YAML file to use !php/const instead of !php/const: for PHP constants. But the former is not able to evaluate the NULL constant.
See the difference between
https://github.com/symfony/symfony/blob/3.4/src/Symfony/Component/Yaml/Inline.php#L697
and
https://github.com/symfony/symfony/blob/3.4/src/Symfony/Component/Yaml/Inline.php#L710

self::parseScalar('null') returns the empty string so constant('') fails.

What should be used in YAML to express PHP's NULL constant?

For XML definitions it is still possible: https://github.com/symfony/framework-bundle/blob/master/Resources/config/workflow.xml#L12