[BUGFIX] Fix PHP 8 warning in TypoLinkCodecService

Added string cast before trimming $typoLink. Changed annotation
to be string|null. With TYPO3 v13 string should be added to the
methods signature and cast removed.
In addition fixed some potential calls that could have sent null
to the method, prophylactically.

Resolves: #99975
Releases: main, 11.5
Change-Id: I8cbafa51d96e3ab35d39855831803897b4ff0035
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77956


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>