Symfony version(s) affected: 3.4.30

Description
Upon update from 3.4.29 to 3.4.30 a new exception is being thrown by framework bundle when save_path is used at framework.session when handler is set to null. This breaks previously working config (site doesn't use sessions).

It was introduced here: #31620

How to reproduce
https://github.com/phpdocker-io/phpdocker.io/blob/master/app/config/config.yml#L30

That was IIRC default config. Site doesn't use sessions.

Possible Solution
Remove the LogicException and instead throw a deprecation warning.