SPIGOT-2540: Add nullability annotations to entire Bukkit API

By: Darkyenus <darkyenus@gmail.com>

paper-api/src/main/java/org/bukkit/entity/Hanging.java

@@ -2,6 +2,7 @@ package org.bukkit.entity;
 
 import org.bukkit.block.BlockFace;
 import org.bukkit.material.Attachable;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * Represents a Hanging entity
@@ -18,5 +19,5 @@ public interface Hanging extends Entity, Attachable {
      * @return False if force was false and there was no block for it to
      *     attach to in order to face the given direction.
      */
-    public boolean setFacingDirection(BlockFace face, boolean force);
+    public boolean setFacingDirection(@NotNull BlockFace face, boolean force);
 }