[BUGFIX] Prevent undefined array key warnings in PageRenderer

Since JS and CSS related properties might be
modified by hooks, corresponding array access
is now secured using null coalescing operators.
This was actually already done for the majority
of those places.

Note: Some places might not make sense to
fallback, e.g. empty css code or empty JS
file paths. However, this should be handled
in a separate patch, and for main only.

Resolves: #97474
Releases: main, 11.5
Change-Id: Ifa396fb23fd3a373a6180e7a9fc06d2a25281171
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74437


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