[BUGFIX] Fix PHP 8 warning in ContentObjectRenderer

Since $conf['externalBlocks.'][$tagName . '.'] can be null, added a null
coalescing check to ensure that $cfg contains a valid value.

Resolves: #99706
Releases: main, 11.5
Change-Id: Ia80bc50504e27eb0155232e1f1eb956f9b88ac79
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77576


Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>