[TASK] FormDataTraverser::getFieldValueRecursive() might return NULL

FormDataTraverser::getFieldValueRecursive() might return NULL or
any other type as well, as it has been provided by the database.
Thus, PHPdoc comments are modified to reflect the possible return
values as well.

Fixes: #53573
Releases: 6.2
Change-Id: Idcdbe21ccdcf694b8ceb5ecf0b4a192ad51c4871
Reviewed-on: https://review.typo3.org/25366
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn