[TASK] Remove unused variable in DataHandler

Initializing the variable `$nativeDateTimeFieldEmptyValue` to
an empty string in the function `checkValueForDatetime` is
superfluous, since the variable is initialized again in the
following if-condition and only used inside the condition
code.

The initialization to an empty string can therefore safely
be removed.

Resolves: #103643
Releases: main
Change-Id: Ifd845228dfb2f5eec2f75c4232602921d9dc964b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83840


Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Andreas Kienast <a.fernandez@scripting-base.de>
Tested-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Reviewed-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Reviewed-by: Jörg Bösche <typo3@joergboesche.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Andreas Kienast <a.fernandez@scripting-base.de>