[BUGFIX] PHP warning on saving TypoScript with t3editor

On saving TypoScript data in the accordant backend module
using t3editor, the AJAX call issues a PHP warning
"Creating default object from empty value". This only
happens if at least PHP 5.4 is used. The reason is,
that an uninitialized and empty variable is used for
object access.

Resolves: #62984
Releasses: 4.5
Change-Id: I7567d61f0a16379db58760dd963f0330dc8ca6c8
Reviewed-on: http://review.typo3.org/34203
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>