Fix missing assignment of playerKey in ServerLogin constructor (#776)

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/ServerLogin.java

@@ -40,6 +40,7 @@ public class ServerLogin implements MinecraftPacket {
 
   public ServerLogin(String username, @Nullable IdentifiedKey playerKey) {
     this.username = Preconditions.checkNotNull(username, "username");
+    this.playerKey = playerKey;
   }
 
   public String getUsername() {