SPIGOT-2540: Add nullability annotations to entire Bukkit API

By: Darkyenus <darkyenus@gmail.com>

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

@@ -1,6 +1,7 @@
 package org.bukkit.material;
 
 import org.bukkit.block.BlockFace;
+import org.jetbrains.annotations.NotNull;
 
 public interface Directional {
 
@@ -9,12 +10,13 @@ public interface Directional {
      *
      * @param face The facing direction
      */
-    public void setFacingDirection(BlockFace face);
+    public void setFacingDirection(@NotNull BlockFace face);
 
     /**
      * Gets the direction this block is facing
      *
      * @return the direction this block is facing
      */
+    @NotNull
     public BlockFace getFacing();
 }