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.
[TASK] Ensure HTTP RequestHandlers always return a PSR-7 Repsonse
This is in preparation for PSR-15 middleware support which
will require PSR-7 RespnseInterface return type declarations
for request handlers.
As TSFE powers a concept of outputting nothing [see isOutputting()]
we need to add NullResponse (which implements the PSR-7 ResponseInterface)
which Core/Bootstrap can detect, to stop invoking header() and echo.
Tested-by: TYPO3com <email@example.com>
Reviewed-by: Mathias Brodala <firstname.lastname@example.org>
Reviewed-by: Benni Mack <email@example.com>
Tested-by: Benni Mack <firstname.lastname@example.org>
Tested-by: Susanne Moog <email@example.com>