[BUGFIX] Prevent PHP warnings in PageRouter

In some situations, "_page" might not be set
in the Route. This patch prevents undefined
array key warnings and also makes the code
more readable.

Resolves: #97369
Releases: main, 11.5
Change-Id: I47920cdeb641b52577590d710d692a91fca7c383
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74295


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>