[BUGFIX] Fix minor issues in functional testing framework

The bootstrap for frontend functional tests has some
minor issues, that are now fixed:

* json_decode returns null not false when not successfull
* Properly set the SERVER_PORT
* Use correct array key in error message
* Avoid notices with relative URLs

Releases: 6.2, 6.3
Resolves: #60413
Change-Id: I3ff07f01729d3403374759edbaa5ddcccd2e889a
Reviewed-on: https://review.typo3.org/31695
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Tested-by: Helmut Hummel <helmut.hummel@typo3.org>