[BUGFIX] Fix PHP warning in TcaMigration

Check if defaultExtras exists before accessing it.

Resolves: #79929
Releases: master
Change-Id: I0a85012b335e10b3744aeb847246e4dabbb23f54
Reviewed-on: https://review.typo3.org/51776


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>