[BUGFIX] Exception on call of setDate

dateOfBirth could be empty if the object get reconstituded.
With this patch this should not happen anymore and so the
exception should not happen anymore.

Resolves: #59014
Releases: 6.2
Change-Id: I0333c858bcc144a2aa23b8757ee8cd1cd478bfbb
Reviewed-on: https://review.typo3.org/30381
Reviewed-by: Sebastian Fischer
Tested-by: Sebastian Fischer