[Feature]: Add “nullmailer” als allowed MTA for email notification

### User story

Allow “nullmailer” as MTA and do not display an error message.

### Game

_No response_

### Linux distro

Debian 12, Debian 11, Ubuntu 20.04, Ubuntu 22.04, Ubuntu 24.04

### Command

command: start

### Further information

*nullmailer* is nice because it is a minimal MTA, just to send email out — and not more than this is needed by LinuxGSM.

I am using *nullmailer* with LinuxGSM already for some years now and it works fine.

However you get the error:

```
Missing dependencies: postfix Run: 'sudo apt update; sudo apt install postfix' as root to install missing dependencies.
```

although sendig emails by LinuxGSM works fine as the `mail` and `mailx` commands can use *nullmailer* as MTA.

See #4871 for a fix.