[BUGFIX] Read TER XML entity value complete

Improve reading the TER XML file while using the php xml extension.
The character data handler could be called multiple times, so we
shouldn't cut previous data away.

Resolves: #83227
Releases: master, 8.7, 7.6
Change-Id: I9e96ade88da6a4709d1f72cd49fe766758d265fe
Reviewed-on: https://review.typo3.org/55058


Reviewed-by: Jigal van Hemert <jigal.van.hemert@typo3.org>
Tested-by: Jigal van Hemert <jigal.van.hemert@typo3.org>