[BUGFIX] Handle URLs withouth a path

If the AJAX API receives a pre-composed URL with a path in front, the
current location of the document is now prepended.

Resolves: #90198
Releases: master
Change-Id: I94dcd12f83016ad1604e64f0e3d9bc6e61be5605
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63030


Tested-by: Susanne Moog <look@susi.dev>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>