[BUGFIX] Illegal string offset 'data' in tx_templavoila_pi1

A string is accesed as array, which might happen if there is
no XML data (or the flexform conversion failed). In that case
the mentioned warning about illegal string offset is shown.

Resolves: #60836
Releases: 2.0.0, 1.9.3
Change-Id: Iea0c7a811464dadae2cab4ff214f6d17f33af9ce
Reviewed-on: http://review.typo3.org/32044
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>