SPIGOT-2540: Add nullability annotations to entire Bukkit API

By: Darkyenus <darkyenus@gmail.com>

paper-api/src/main/java/org/bukkit/material/Attachable.java

@@ -1,6 +1,7 @@
 package org.bukkit.material;
 
 import org.bukkit.block.BlockFace;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * Indicates that a block can be attached to another block
@@ -12,5 +13,6 @@ public interface Attachable extends Directional {
      *
      * @return BlockFace attached to
      */
+    @NotNull
     public BlockFace getAttachedFace();
 }