SPIGOT-2540: Add nullability annotations to entire Bukkit API

By: Darkyenus <darkyenus@gmail.com>

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

@@ -1,6 +1,7 @@
 package org.bukkit.entity;
 
 import org.bukkit.util.Vector;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * Represents a Fireball.
@@ -12,13 +13,14 @@ public interface Fireball extends Projectile, Explosive {
      *
      * @param direction the direction this fireball is flying toward
      */
-    public void setDirection(Vector direction);
+    public void setDirection(@NotNull Vector direction);
 
     /**
      * Retrieve the direction this fireball is heading toward
      *
      * @return the direction
      */
+    @NotNull
     public Vector getDirection();
 
 }