[BUGFIX] Make section title in SytemEmail layout optional

The argument `optional="true"` must be added to the plain text layout as
well - just as in the HTML version.

Resolves: #90763
Releases: master
Change-Id: I61ff71fd0780e2fdffba1c6b190fd60d1dc01581
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63726


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>