Support stand alone STUN server mode

Running in stand alone STUN server does not start
due to packet conn config requiring relay address config
during validation.

To enable that mode, bypass validation if relay address config
in packet conn config is nil.

Note that it is still validated in `ListenerConfig`.

server_config.go

@@ -57,11 +57,14 @@ func (c *PacketConnConfig) validate() error {
 	if c.PacketConn == nil {
 		return errConnUnset
 	}
-	if c.RelayAddressGenerator == nil {
-		return errRelayAddressGeneratorUnset
+
+	if c.RelayAddressGenerator != nil {
+		if err := c.RelayAddressGenerator.Validate(); err != nil {
+			return err
+		}
 	}
 
-	return c.RelayAddressGenerator.Validate()
+	return nil
 }
 
 // ListenerConfig is a single net.Listener to accept connections on. This will be used for TCP, TLS and DTLS listeners