[BUGFIX] Do not unset() PageRenderer class members

Instead reset them to a default value.

Resolves: #57920
Releases: 6.2
Change-Id: I900cbd46b5573a84133efe1cf9fbf29d483e8096
Reviewed-on: https://review.typo3.org/29469
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind