Support overriding the jump host to an empty one (none)

[bryant1410]

I have a situation in which I'd like to ignore the jump host configuration set in my SSH config when using the ET client, but not when using the SSH client. I don't find a way of doing this. --jumphost= (empty) doesn't seem to work. I also checked the code and it makes sense that an empty one doesn't work:

https://github.com/MisterTea/EternalTerminal/blob/0aa443c2141069cc87cd4694fd644fea2444a938/src/terminal/TerminalClientMain.cpp#L268

By looking at the code, I couldn't find a way to do this. It'd be nice to distinguish between not passing this option at all and setting it to empty. As a reference point, the SSH config syntax sometimes allows setting None to certain configs. E.g., see https://stackoverflow.com/a/62015832/1165181