CA is always required for OpenVPN client

[ghost]

https://github.com/keeshux/tunnelkit/blob/d0a46fe20e536571ea83625f8ef6437fa53161f2/TunnelKit/Sources/Core/TLSBox.m#L118

The official OpenVPN client requires a CA to be set in the client configuration file. Without setting a CA the OpenVPN client gives an error and refuses to even try to connect.

1. The CA certificate MUST be always set in the client configuration, no reason to have an if for this;
2. The server certificate sent by the VPN server when establishing the TLS channel MUST be verified to be signed by this CA.