[BUGFIX] Correct uri in functional test

Handing over '/' as $uri to InternalRequest() in functional
tests triggers an ugly fallback mechanism that changes this to
'http://localhost/' within the FE call.
Not handing over any $uri string at all defaults to
'http://localhost/' too, but at an much earlier and more
transparent point.
The patch streamlines two places that used the above method.

Resolves: #92877
Releases: master, 10.4
Change-Id: Ic8e8e58315efeaf49298737f549bb5955520f7f8
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/66700


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>