[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