[BUGFIX] CKEditor: Avoid JS error when opening an existing link

When opening an existing link by double-clicking on the
a tag element in CKEditor the popup should open, however
if no additional attributes are registered, JS throws an
error.

Resolves: #79818
Releases: master
Change-Id: I9f9b838862ab76d21161d4962f75f88d70c44cdd
Reviewed-on: https://review.typo3.org/51691


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Frans Saris <franssaris@gmail.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>