Make password optional.

[fernvenue]

Is your feature request related to a problem? Please describe.
As we know, shadowsocks-windows already support plain/none cipher from 4.4.1.0, and when we use plain/none actually we don't really need a password, but we still have to write something here even if it will do nothing.

Describe the solution you'd like
I think making password optional will be a good choice, let's just leave it blank.

Describe alternatives you've considered
When password is required, users may mistakenly believe that they are using a password to authenticate.

Additional context
By the way, shadowsocks-rust already do that: shadowsocks/shadowsocks-rust#838.