[BUGFIX] Fix backtracking error in UrlSoftReferenceParser

In case the URL is at the very end of a string and there are no space
characters that follow, the preg_split has failed with a backtracking
error. By declaring the last capturing group as optional, this error is
fixed.

Resolves: #94920
Releases: master
Change-Id: Id846840d910f38612d9b9185b4ed307a62422d4b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70676


Reviewed-by: Frank Nägler <frank.naegler@typo3.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Frank Nägler <frank.naegler@typo3.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>