[BUGFIX] Fix the return type annotations of JsonView

transformValue will return the argument (mixed) untransformed if it is no
array or object and thus needs to return mixed as well.

transformObject will return a string for DateTime and hence needs to have
string as an additional return type.

Resolves: #84570
Releases: master, 8.7
Change-Id: I4d73f060eebb20b6f521fa685eca99acb688c702
Reviewed-on: https://review.typo3.org/56499


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>