[BUGFIX] Fix side effect for new class instantiation

With commit 6eb7a548 performance optimized class
instantiation code has been committed. This code
removed the side effect of a reflection exception
being thrown when a not existing class is instantiated.

Code in ContentObjectRenderer relied on this side
effect, so we have to fix that and properly test
if the class exists instead.

Additionally this change adds some more comments
to the new instantiation code that has been forgotten
in the last commit.

Resolves: #54425
Releases: 6.2
Change-Id: I8962434d60f80daf77ccdce7a8148e26f8fee267
Reviewed-on: https://review.typo3.org/26440
Reviewed-by: Marcin S?gol
Tested-by: Marcin S?gol
Reviewed-by: Markus Klein
Reviewed-by: Stefan Neufeind
Tested-by: Anja Leichsenring
Reviewed-by: Anja Leichsenring
Tested-by: Tobias Liegl
Reviewed-by: Frans Saris
Tested-by: Frans Saris
Reviewed-by: Dmitry Dulepov
Tested-by: Dmitry Dulepov
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer