[BUGFIX] Force string casting of $parserList

The method `BackendUtility::explodeSoftRefParserList()` checks whether
`$parserList` is an empty string. This check may fail, as the variable may
be null instead. `$parserList` is now casted to a string, which converts
null to an empty string.

Resolves: #78858
Releases: master, 7.6
Change-Id: Ia833fd64f3349c5856a879e917bb3cd6f581caa0
Reviewed-on: https://review.typo3.org/50838


Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>