SPIGOT-2540: Add nullability annotations to entire Bukkit API

By: Darkyenus <darkyenus@gmail.com>

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

@@ -3,6 +3,7 @@ package org.bukkit.block;
 import org.bukkit.DyeColor;
 import org.bukkit.Nameable;
 import org.bukkit.loot.Lootable;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * Represents a captured state of a ShulkerBox.
@@ -14,5 +15,6 @@ public interface ShulkerBox extends Container, Nameable, Lootable {
      *
      * @return the {@link DyeColor} of this ShulkerBox
      */
+    @NotNull
     public DyeColor getColor();
 }