[BUGFIX] Throw exception if encryption key is not set

The encryption key is a vital part to ensure security
of TYPO3. Not having it set (by accident) can lead
to severe security issues.

Therefore enforce it to be set and throw an exception
in case it isn't, instead of just reporting a warning.

Releases: master, 8.7, 7.6
Resolves: #82169
Change-Id: I79c73558d0968ae89a2867d6d5264530f434e1a3
Reviewed-on: https://review.typo3.org/53724


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>