Optimize entity tracker passenger checks (#6361)

When a ServerEntity instance is first constructed, it uses a pre-Java 9 empty collection,
but passengers use Guava ImmutableList. When comparing the two collections via Object#equals
we miss out on an opportunity to reduce the check to little more than a reference equality
check (which is very fast).

patches/server/Optimize-entity-tracker-passenger-checks.patch

@@ -0,0 +1,19 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Andrew Steinborn <git@steinborn.me>
+Date: Sun, 8 Aug 2021 00:52:54 -0400
+Subject: [PATCH] Optimize entity tracker passenger checks
+
+
+diff --git a/src/main/java/net/minecraft/server/level/ServerEntity.java b/src/main/java/net/minecraft/server/level/ServerEntity.java
+index 0000000000000000000000000000000000000000..0000000000000000000000000000000000000000 100644
+--- a/src/main/java/net/minecraft/server/level/ServerEntity.java
++++ b/src/main/java/net/minecraft/server/level/ServerEntity.java
+@@ -0,0 +0,0 @@ public class ServerEntity {
+         this.trackedPlayers = trackedPlayers;
+         // CraftBukkit end
+         this.ap = Vec3.ZERO;
+-        this.lastPassengers = Collections.emptyList();
++        this.lastPassengers = com.google.common.collect.ImmutableList.of(); // Paper - optimize passenger checks
+         this.level = worldserver;
+         this.broadcast = consumer;
+         this.entity = entity;