[BUGFIX] LinkService should identify urls starting with //

Urls starting with `//` should have a proper fallback to
avoid warnings e.g. in InputLinkElement that this type of url
is unknown.

Resolves: #90171
Releases: master, 9.5
Change-Id: Ic3910f98c3993fb5db523ba1b360eabb49bf0696
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63543


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>