…guration values

…pping configuration values (andrew-demb)This PR was squashed before being merged into the 5.4 branch.Discussion----------[FrameworkBundle] Allow to specify `null` for exception mapping configuration values| Q             | A| ------------- | ---| Branch?       | 5.4| Bug fix?      | yes| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->| Tickets       || License       | MIT| Doc PR        |<!--Replace this notice by a short README for your feature/bugfix.This will help reviewers and should be a good start for the documentation.Additionally (seehttps://symfony.com/releases): - Always add tests and ensure they pass. - Bug fixes must be submitted against the lowest maintained branch where they apply   (lowest branches are regularly merged to upper ones so they get the fixes too.) - Features and deprecations must be submitted against the latest branch. - Changelog entry should followhttps://symfony.com/doc/current/contributing/code/conventions.html#writing-a-changelog-entry - Never break backward compatibility (seehttps://symfony.com/bc).-->`null` should be allowed because it is stated as allowed in the configuration info [1][2]Marked as bugfix due to an existing mismatch between config description and behavior.[1]https://github.com/symfony/symfony/blob/a2f27add28f7c2c61ef4cfb4e97833509ced0fc0/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Configuration.php#L1219-L1220[2]https://github.com/symfony/symfony/blob/a2f27add28f7c2c61ef4cfb4e97833509ced0fc0/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Configuration.php#L1227-L1228Commits-------1de8f3a9ff [FrameworkBundle] Allow to specify `null` for exception mapping configuration values