Switching the role throws an Exception in TYPO3 v8

[denislorch]

The RoleSwitcher::switchRoleAction() method throws an exception because the \TYPO3\CMS\Core\Http\NullResponse class is not available in TYPO3 v8, but in TYPO3 v9/10.
This commit breaks it.

[baschny]

As a suggestion: adding a test for this class would avoid this kind of incompatibility to slip in a patch level release. Either keep it compatible with v8 and v10 or drop support for v8 (in composer.json). As "3.1.0-3.1.2" are already tagged with "compatible with 8/9/10", it would be smoother if it could be reestablished in a "3.1.3", and break it in the next minor (3.2.0).