SPIGOT-7872: Fix crash with event-modified teleports

By: md_5 <git@md-5.net>

paper-server/nms-patches/net/minecraft/server/network/PlayerConnection.patch

@@ -573,7 +573,7 @@
 +        this.cserver.getPluginManager().callEvent(event);
 +
 +        if (event.isCancelled() || !to.equals(event.getTo())) {
-+            set.clear(); // Can't relative teleport
++            set = Collections.emptySet(); // Can't relative teleport
 +            to = event.isCancelled() ? event.getFrom() : event.getTo();
 +            d0 = to.getX();
 +            d1 = to.getY();