[TASK] Log warning on invalid charset

This gives users a clear hint if e.g. config.metaCharset contains
an invalid value instead of showing a blank page.

Change-Id: I9cdb37cc7e61af10e2d1985ef43f488e53b17a48
Resolves: #82852
Releases: master, 8.7
Reviewed-on: https://review.typo3.org/54503


Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Tested-by: Pawel Cieslik <p.cieslik@macopedia.pl>
Reviewed-by: Pawel Cieslik <p.cieslik@macopedia.pl>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>