[BUGFIX] Avoid autoloading of non-objects in DataMapper

While persisting an object, the DataMapper is asked for the plain value
of a property, the TypeHandlingUtility::isCoreType() method is using
"is_subclass_of()" to check if the given input is an instance of 
\TYPO3\CMS\Core\Type\TypeInterface.
Unfortunately, when passing a string into "is_subclass_of" it tries to 
instantiate an object of that name. This will trigger the class autoloader.
Even worse, when the string contains a special char (like an umlaut) the
class cache will throw an exception (#1233057752). Thus persisting an object
with special chars in a property value is not possible at the moment.

This change fixes the issue by calling TypeHandlingUtility::isCoreType() only
if $input is an object.

Resolves: #53425
Releases: 6.3
Change-Id: I8b97499f2d90e26ba2a5dec411de266948fdf005
Reviewed-on: http://review.typo3.org/25349
Reviewed-by: Tobias Liebig <tobias.liebig@typo3.org>
Tested-by: Tobias Liebig <tobias.liebig@typo3.org>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>