[BUGFIX] fallbackRendering is always called

When rendering textpic, the content object rendering is always called for
the Typoscript setting fallbackRendering which is not always defined.

Change-Id: I2ebfffd2ece8022136c4fcc8d0e12d552486dd32
Fixes: #44732
Releases: 6.1, 6.0, 4.7, 4.5
Reviewed-on: https://review.typo3.org/17661
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn