SPIGOT-5853: DragonBattle#getEndPortalLocation() throws NPE on new world

* Add method to generate the end portal

nms-patches/EnderDragonBattle.patch

@@ -0,0 +1,33 @@
+--- a/net/minecraft/server/EnderDragonBattle.java
++++ b/net/minecraft/server/EnderDragonBattle.java
+@@ -245,7 +245,7 @@
+     }
+ 
+     @Nullable
+-    private ShapeDetector.ShapeDetectorCollection j() {
++    public ShapeDetector.ShapeDetectorCollection j() { // PAIL private -> public, rename getExitPortalShape()
+         int i;
+         int j;
+ 
+@@ -376,10 +376,10 @@
+ 
+     private void a(BlockPosition blockposition) {
+         this.world.triggerEffect(3000, blockposition, 0);
+-        WorldGenerator.END_GATEWAY.b((WorldGenFeatureConfiguration) WorldGenEndGatewayConfiguration.a()).a(this.world, this.world.getStructureManager(), this.world.getChunkProvider().getChunkGenerator(), new Random(), blockposition);
++        WorldGenerator.END_GATEWAY.b(WorldGenEndGatewayConfiguration.a()).a(this.world, this.world.getStructureManager(), this.world.getChunkProvider().getChunkGenerator(), new Random(), blockposition); // CraftBukkit - decompile error
+     }
+ 
+-    private void a(boolean flag) {
++    public void a(boolean flag) { // PAIL private -> public, rename generateExitPortal()
+         WorldGenEndTrophy worldgenendtrophy = new WorldGenEndTrophy(flag);
+ 
+         if (this.exitPortalLocation == null) {
+@@ -388,7 +388,7 @@
+             }
+         }
+ 
+-        worldgenendtrophy.b((WorldGenFeatureConfiguration) WorldGenFeatureConfiguration.k).a(this.world, this.world.getStructureManager(), this.world.getChunkProvider().getChunkGenerator(), new Random(), this.exitPortalLocation);
++        worldgenendtrophy.b(WorldGenFeatureConfiguration.k).a(this.world, this.world.getStructureManager(), this.world.getChunkProvider().getChunkGenerator(), new Random(), this.exitPortalLocation); // CraftBukkit - decompile error
+     }
+ 
+     private EntityEnderDragon o() {

src/main/java/org/bukkit/craftbukkit/boss/CraftDragonBattle.java

@@ -30,9 +30,23 @@ public class CraftDragonBattle implements DragonBattle {
 
     @Override
     public Location getEndPortalLocation() {
+        if (handle.exitPortalLocation == null) {
+            return null;
+        }
+
         return new Location(handle.world.getWorld(), handle.exitPortalLocation.getX(), handle.exitPortalLocation.getY(), handle.exitPortalLocation.getZ());
     }
 
+    @Override
+    public boolean generateEndPortal(boolean withPortals) {
+        if (handle.exitPortalLocation != null || handle.j() != null) {
+            return false;
+        }
+
+        this.handle.a(withPortals);
+        return true;
+    }
+
     @Override
     public boolean hasBeenPreviouslyKilled() {
         return handle.isPreviouslyKilled();