fix: problem with PluginMessageEvents for configuration phase (PaperMC#1517)

Author: ST3V1K

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ClientConfigSessionHandler.java

@@ -134,7 +134,7 @@ public boolean handle(final PluginMessagePacket packet) {
       ChannelIdentifier id = this.server.getChannelRegistrar().getFromId(packet.getChannel());
 
       if (id == null) {
-        serverConn.getPlayer().getConnection().write(packet.retain());
+        serverConn.ensureConnected().write(packet.retain());
         return true;
       }