Epic #83894: PSR-15 Initiative
HTTP RequestHandlers should always return a PSR-7 ResponseInterface, not NULL
The PSR-15 middleware interfaces  require that RequestHandlers always return a Response.
In order to support PSR-15 middleware (at some point), TYPO3 request handlers should do the same.
A NULL return value should rather be replaced with a PSR-7 Response that's either ignored by Core\Bootstrap or a Response with a 200 status code and an empty body.