SPIGOT-2540: Add nullability annotations to entire Bukkit API

By: Darkyenus <darkyenus@gmail.com>

paper-api/src/main/java/org/bukkit/advancement/Advancement.java

@@ -2,6 +2,7 @@ package org.bukkit.advancement;
 
 import java.util.Collection;
 import org.bukkit.Keyed;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * Represents an advancement that may be awarded to a player. This class is not
@@ -14,5 +15,6 @@ public interface Advancement extends Keyed {
      *
      * @return a unmodifiable copy of all criteria
      */
+    @NotNull
     Collection<String> getCriteria();
 }