Allow enabling sand duping (#10191)

Because this exploit has been widely known for years and has not been fixed by Mojang, we decided that it was worth allowing people to toggle it on/off due to how easy it is to make it configurable.

It should be noted that this decision does not promise all future exploits will be configurable.

patches/server/0491-Empty-commands-shall-not-be-dispatched.patch

@@ -0,0 +1,18 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Mariell Hoversholm <proximyst@proximyst.com>
+Date: Wed, 6 Jan 2021 23:38:43 +0100
+Subject: [PATCH] Empty commands shall not be dispatched
+
+
+diff --git a/src/main/java/net/minecraft/commands/Commands.java b/src/main/java/net/minecraft/commands/Commands.java
+index d39743d9626eb01c942194387dafb1106f13601b..0741c0f85536c5188d8552e999943a9c771a548e 100644
+--- a/src/main/java/net/minecraft/commands/Commands.java
++++ b/src/main/java/net/minecraft/commands/Commands.java
+@@ -287,6 +287,7 @@ public class Commands {
+         command = event.getCommand();
+ 
+         String[] args = command.split(" ");
++        if (args.length == 0) return; // Paper - empty commands shall not be dispatched
+ 
+         String cmd = args[0];
+         if (cmd.startsWith("minecraft:")) cmd = cmd.substring("minecraft:".length());