[TASK] Use instanceof comparison instead of string comparison

This allows the ViewHelper to render elements which are extending
the model.

Resolves: #52469
Releases: 6.2, 6.1, 6.0
Change-Id: If4b9b3019b44e38a5a25cb74ff993e572464bb9d
Reviewed-on: https://review.typo3.org/24257
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Markus Klein
Tested-by: Markus Klein