[BUGFIX] Clean up DataHandler unit tests

This fixes wrong input configuration, wrong method calls and adds
some missing test cases.

- The first parameter of assertSame expects the expected value. All
occurrences in DataHandlerTest, which had them mixed up, are corrected.

- TCA configuration "type => input" corrected.

- Missing test cases with empty string as input added.

Resolves: #94682
Releases: master
Change-Id: If31ad11bb1e958fd24b1cbdb7d525d056fe3111b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70175


Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Benni Mack <benni@typo3.org>