SPIGOT-7446: BlockState#update not updating a spawner's type to null

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

paper-server/src/main/java/org/bukkit/craftbukkit/block/CraftCreatureSpawner.java

@@ -32,7 +32,7 @@ public class CraftCreatureSpawner extends CraftBlockEntityState<TileEntityMobSpa
     public void setSpawnedType(EntityType entityType) {
         if (entityType == null) {
             this.getSnapshot().getSpawner().spawnPotentials = SimpleWeightedRandomList.empty(); // need clear the spawnPotentials to avoid nextSpawnData being replaced later
-            this.getSnapshot().getSpawner().nextSpawnData = null;
+            this.getSnapshot().getSpawner().nextSpawnData = new MobSpawnerData();
             return;
         }
         Preconditions.checkArgument(entityType != EntityType.UNKNOWN, "Can't spawn EntityType %s from mob spawners!", entityType);