[BUGFIX] Faulty check for missing SMTP port

Check also for null port to avoid a fatal error.

Resolves: #31998
Releases: 6.2, 6.1, 6.0, 4.7, 4.5
Change-Id: Ibf45b3c0783a70e5afba33f90d1d8e05f76834cf
Reviewed-on: https://review.typo3.org/23517
Reviewed-by: Markus Klein
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind