[BUGFIX] Allow digit only strings in EmailFinisher

Since it's valid to use a digit-only subject or title, those
options are now properly cast to a string, preventing
TypeError exceptions when passing them to type-hinted
functions. This happened due to PHP converting digit-only
strings to integers.

Resolves: #100582
Releases: main, 11.5
Change-Id: I30c543b64f41b91ed64edabdb4bd2d9720f24e51
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78628


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Oliver Bartsch <bo@cedev.de>