[BUGFIX] CKEditor: Make link editable

Fixes the TypeError

htmlspecialchars() expects parameter 1 to be string, null given

Resolves: #82787
Releases: master,8.7
Change-Id: I38a689022ed45c34a8ae78c046abe51803eb0f4c
Reviewed-on: https://review.typo3.org/54415


Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Henrik Elsner <helsner@dfau.de>
Tested-by: Henrik Elsner <helsner@dfau.de>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Tested-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>