[TASK] Remove superflous fallback guard in ContentObjectRenderer

In `ContentObjectRenderer->getImgResource()` null coalescing operator
has been used to guard type errors. That has been guarded by a early
array check and ensurance, therefore the guards are obselete. Remove
them !

Used command(s):

> Build/Scripts/runTests.sh -s phpstanGenerateBaseline

Resolves: #101982
Releases: main, 12.4
Change-Id: Ic178e29a5d99ded62738707dc285f2c031330fb3
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81119


Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>