SPIGOT-2540: Add nullability annotations to entire Bukkit API

By: Darkyenus <darkyenus@gmail.com>

paper-api/src/main/java/org/bukkit/block/Dispenser.java

@@ -3,6 +3,7 @@ package org.bukkit.block;
 import org.bukkit.Nameable;
 import org.bukkit.loot.Lootable;
 import org.bukkit.projectiles.BlockProjectileSource;
+import org.jetbrains.annotations.Nullable;
 
 /**
  * Represents a captured state of a dispenser.
@@ -18,6 +19,7 @@ public interface Dispenser extends Container, Nameable, Lootable {
      * @return a BlockProjectileSource if valid, otherwise null
      * @throws IllegalStateException if this block state is not placed
      */
+    @Nullable
     public BlockProjectileSource getBlockProjectileSource();
 
     /**