[BUGFIX] Empty subject deactivates mailing

[mhirdes]

Like it is described in the flexform, an empty subject should deactivate the mailing. Currently the users gets an error, that the mail could not be send.

[einpraegsam]

Please describe exactly how to reproduce the issue (what kind of configuration I have to do to see the same error) that you want to fix with that PR.

[mhirdes]

Sometimes the customers only want to save the answers in the DB, not to send a mail. But this they want only for some forms. So the settings.receiver.enable = 1 and settings.sender.enable = 1, but for this specific form, the mailing should be disabled. In the Flexform it is described, that if you leave the subject empty, the mailing will be disabled. But currently it will not be disabled. The user gets an error.