[BUGFIX] Fix invalid array access in Mail\TransportFactory

Even for a broken/imcomplete configuration, we want to have the
exception that complains about the incomplete configuration instead
of a `TypeError`.

Also add some type casts to have predictable types in more cases.

Resolves: #100431
Related: #100249
Releases: main, 11.5
Change-Id: If92af8f66bab7d311cbddc3178bc515b63381741
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78452


Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>