[BUGFIX] Handle empty absRefPrefix in PageLinkBuilder

Using empty values for absRefPrefix as needle in strpos() will
throw a warning. In order to avoid that, it's now checked more
explicitly if the prefixed slash originally was absRefPrefix.

Resolves: #84676
Releases: master
Change-Id: I7515e8d283b2d68e39f7a0451432d453471088f0
Reviewed-on: https://review.typo3.org/56616


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>