Ensure empty (un)register packets are never sent.

Bukkit 1.13+, in particular, doesn't seem to like it very much. This was
also a bug in ViaVersion.

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

@@ -1,6 +1,7 @@
 package com.velocitypowered.proxy.connection.client;
 
 import static com.velocitypowered.api.network.ProtocolVersion.MINECRAFT_1_13;
+import static com.velocitypowered.proxy.protocol.util.PluginMessageUtil.constructChannelsPacket;
 
 import com.velocitypowered.api.event.connection.PluginMessageEvent;
 import com.velocitypowered.api.event.player.PlayerChatEvent;
@@ -72,11 +73,12 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
   public void activated() {
     Collection<String> channels = server.getChannelRegistrar().getChannelsForProtocol(player
         .getProtocolVersion());
-    PluginMessage register = PluginMessageUtil.constructChannelsPacket(player.getProtocolVersion(),
-        channels);
+    if (!channels.isEmpty()) {
+      PluginMessage register = constructChannelsPacket(player.getProtocolVersion(), channels);
       player.getMinecraftConnection().write(register);
       player.getKnownChannels().addAll(channels);
     }
+  }
 
   @Override
   public boolean handle(KeepAlive packet) {
@@ -366,8 +368,7 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
     // Tell the server about this client's plugin message channels.
     ProtocolVersion serverVersion = serverMc.getProtocolVersion();
     if (!player.getKnownChannels().isEmpty()) {
-      serverMc.delayedWrite(PluginMessageUtil.constructChannelsPacket(serverVersion,
-          player.getKnownChannels()));
+      serverMc.delayedWrite(constructChannelsPacket(serverVersion, player.getKnownChannels()));
     }
 
     // If we had plugin messages queued during login/FML handshake, send them now.

proxy/src/main/java/com/velocitypowered/proxy/protocol/util/PluginMessageUtil.java

@@ -93,6 +93,11 @@ public class PluginMessageUtil {
     checkNotNull(message, "message");
     checkArgument(isRegister(message) || isUnregister(message), "Unknown channel type %s",
             message.getChannel());
+    if (message.getData().length == 0) {
+      // If we try to split this, we will get an one-element array with the empty string, which
+      // has caused issues with 1.13+ compatibility. Just return an empty list.
+      return ImmutableList.of();
+    }
     String channels = new String(message.getData(), StandardCharsets.UTF_8);
     return ImmutableList.copyOf(channels.split("\0"));
   }
@@ -103,10 +108,10 @@ public class PluginMessageUtil {
    * @param channels the channels to register
    * @return the plugin message to send
    */
-
   public static PluginMessage constructChannelsPacket(ProtocolVersion protocolVersion,
                                                       Collection<String> channels) {
     Preconditions.checkNotNull(channels, "channels");
+    Preconditions.checkArgument(channels.size() > 0, "no channels specified");
     String channelName = protocolVersion.compareTo(ProtocolVersion.MINECRAFT_1_13) >= 0
         ? REGISTER_CHANNEL : REGISTER_CHANNEL_LEGACY;
     PluginMessage message = new PluginMessage();