Configuration sourcing from ENV vars breaks optional fields

[valeriansaliou]

The following attribute: #[serde(deserialize_with = "env_var::opt_str")] prevents the optional configuration options to be treated as optional and omitted in the configuration file passed to Sonic.

This is a regression caused by the introduction of the "${env.VAR}" syntax.

Trace when no auth_password is set (which should work):

⋊> ~/C/c/c/development on master ◦ sonic -c sonic.conf                                                                         15:03:13
thread 'main' panicked at 'syntax error in config file: Error { inner: ErrorInner { kind: Custom, line: None, col: 0, message: "missing field `auth_password`", key: ["channel"] } }', src/libcore/result.rs:997:5
note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.