[BUGFIX] PHP 8.0 warning from ContentObjectRenderer

TypoScript options config.spamProtectEmailAddresses_atSubst
and config.spamProtectEmailAddresses_lastDotSubst may not
be set. Use null coalescing operator to sanitize access.

Link/EmailViewHelperTest stumbles here and also needs a
proper initialization of $GLOBALS['BE_USER'].

Resolves: #95532
Releases: master
Change-Id: I8169229c1a31c1243cf2c4fc4321d015f532dc12
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71494


Tested-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Benni Mack <benni@typo3.org>