[BUGFIX] Do not reset Page Renderer on partial rendering

Currently the Page Renderer is reset when calling
the render() method, even if only one part (header
or footer) is rendered, which causes a second call
to fail, when rendering another part.
This happens e.g in DocumentTemplate.php where
the render() method is called two times. Because of
the reset, the second call starts pageRenderer with
empty properties.

Solution is to only call reset if everything is rendered.

Resolves: #57294
Releases: 6.2
Change-Id: Ib4800e37ba0760617de71f9ca1181d1664463e33
Reviewed-on: https://review.typo3.org/28770
Reviewed-by: Nicole Cordes
Tested-by: Nicole Cordes
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind