Relookup Entity Save ID if was null during precache

Should fix #1280

Citizens hijacks entity map, and im guessing under the right conditions
the result might actually be null during entity creation

Pre the cache patch, the id is looked up on save, so it was fine.

Now, if its null and the save ID is requested, we will try to look
it up again and cache it if found.

Spigot-Server-Patches/Optional-TNT-doesn-t-move-in-water.patch

@@ -5,7 +5,7 @@ Subject: [PATCH] Optional TNT doesn't move in water
 
 
 diff --git a/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java b/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java
-index 067cb233e..06acdaaf0 100644
+index 067cb233e4..06acdaaf04 100644
 --- a/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java
 +++ b/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java
 @@ -0,0 +0,0 @@ package com.destroystokyo.paper;
@@ -32,7 +32,7 @@ index 067cb233e..06acdaaf0 100644
 +    }
  }
 diff --git a/src/main/java/net/minecraft/server/Entity.java b/src/main/java/net/minecraft/server/Entity.java
-index 9b01c23e0..0a62ebf4a 100644
+index 0456cda937..2fbe17ce9d 100644
 --- a/src/main/java/net/minecraft/server/Entity.java
 +++ b/src/main/java/net/minecraft/server/Entity.java
 @@ -0,0 +0,0 @@ public abstract class Entity implements ICommandListener, KeyedObject { // Paper
@@ -60,7 +60,7 @@ index 9b01c23e0..0a62ebf4a 100644
      }
  
 diff --git a/src/main/java/net/minecraft/server/EntityTNTPrimed.java b/src/main/java/net/minecraft/server/EntityTNTPrimed.java
-index bb0904f86..50811852a 100644
+index bb0904f865..50811852a3 100644
 --- a/src/main/java/net/minecraft/server/EntityTNTPrimed.java
 +++ b/src/main/java/net/minecraft/server/EntityTNTPrimed.java
 @@ -0,0 +0,0 @@ public class EntityTNTPrimed extends Entity {