Fix PersistentDataContainer contract violation (_->!null returns null) (#3990)

Spigot-Server-Patches/API-to-get-a-BlockState-without-a-snapshot.patch

@@ -16,6 +16,23 @@ diff --git a/src/main/java/net/minecraft/server/TileEntity.java b/src/main/java/
 index 0000000000000000000000000000000000000000..0000000000000000000000000000000000000000 100644
 --- a/src/main/java/net/minecraft/server/TileEntity.java
 +++ b/src/main/java/net/minecraft/server/TileEntity.java
+@@ -0,0 +0,0 @@ public abstract class TileEntity implements KeyedObject { // Paper
+     public TileEntity(TileEntityTypes<?> tileentitytypes) {
+         this.position = BlockPosition.ZERO;
+         this.tileType = tileentitytypes;
++        persistentDataContainer = new CraftPersistentDataContainer(DATA_TYPE_REGISTRY); // Paper - always init
+     }
+ 
+     // Paper start
+@@ -0,0 +0,0 @@ public abstract class TileEntity implements KeyedObject { // Paper
+     public void load(IBlockData iblockdata, NBTTagCompound nbttagcompound) {
+         this.position = new BlockPosition(nbttagcompound.getInt("x"), nbttagcompound.getInt("y"), nbttagcompound.getInt("z"));
+         // CraftBukkit start - read container
+-        this.persistentDataContainer = new CraftPersistentDataContainer(DATA_TYPE_REGISTRY);
++        this.persistentDataContainer.clear(); // Paper - clear instead of reinit
+ 
+         NBTTagCompound persistentDataTag = nbttagcompound.getCompound("PublicBukkitValues");
+         if (persistentDataTag != null) {
 @@ -0,0 +0,0 @@ public abstract class TileEntity implements KeyedObject { // Paper
      }
  
@@ -130,3 +147,18 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
      }
  
      @Override
+diff --git a/src/main/java/org/bukkit/craftbukkit/persistence/CraftPersistentDataContainer.java b/src/main/java/org/bukkit/craftbukkit/persistence/CraftPersistentDataContainer.java
+index 0000000000000000000000000000000000000000..0000000000000000000000000000000000000000 100644
+--- a/src/main/java/org/bukkit/craftbukkit/persistence/CraftPersistentDataContainer.java
++++ b/src/main/java/org/bukkit/craftbukkit/persistence/CraftPersistentDataContainer.java
+@@ -0,0 +0,0 @@ public final class CraftPersistentDataContainer implements PersistentDataContain
+     public Map<String, Object> serialize() {
+         return (Map<String, Object>) CraftNBTTagConfigSerializer.serialize(toTagCompound());
+     }
++
++    // Paper start
++    public void clear() {
++        this.customDataTags.clear();
++    }
++    // Paper end
+ }