[BUGFIX] PHP Warning related to __extraData

Fixes: #63987
Related: #61978
Change-Id: Ie16b4b75b1ba81437a3a26210eff44b2a0e66616
Reviewed-on: http://review.typo3.org/35583
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Tested-by: Xavier Perseguers <xavier@typo3.org>