[BUGFIX] Respect cached page config

On a cached page, initialise the page config only if it does not exist
yet. Merge the cached config with the template config to get the same
array as on a uncached page.

Releases: master, 6.2
Resolves: #66605
Change-Id: Idee4682cbfe5a8fb5f9522fc2f09e1c9bdb81396
Reviewed-on: http://review.typo3.org/38958


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Susanne Moog <typo3@susannemoog.de>
Tested-by: Susanne Moog <typo3@susannemoog.de>