[BUGFIX] Do not try to parse non-strings as XML in DataHandler

If the flexforms data is null, there is no point trying to parse
that as XML. This avoids type errors when `GeneralUtility` gets
native parameter type declarations for `GU::xml2array`, and it
should also slightly increase performance.

Also fix the corresponding PHPDoc type annotation to reflect
that a parameter in reality needs to be nullable.

Resolves: #101891
Releases: main, 12.4, 11.5
Change-Id: I39efc1d26300634c0bed74aee1f7ebd444acb8c0
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81059


Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>