[BUGFIX] Fix runtime-error in traverseFlexFormXMLData

Fix that run-time error will be thrown if
$dataStructureArray['sheets'] is not an array.

Fixes: #79426
Releases: master
Change-Id: I9b7549a5b79d0d5b8b2e27df407879812c1aa491
Reviewed-on: https://review.typo3.org/51399


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>