SPIGOT-2540: Add nullability annotations to entire Bukkit API

By: Darkyenus <darkyenus@gmail.com>

paper-api/src/main/java/org/bukkit/event/Event.java

@@ -1,6 +1,8 @@
 package org.bukkit.event;
 
+import org.bukkit.plugin.Plugin;
 import org.bukkit.plugin.PluginManager;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * Represents an event.
@@ -40,6 +42,7 @@ public abstract class Event {
      *
      * @return name of this event
      */
+    @NotNull
     public String getEventName() {
         if (name == null) {
             name = getClass().getSimpleName();
@@ -47,6 +50,7 @@ public abstract class Event {
         return name;
     }
 
+    @NotNull
     public abstract HandlerList getHandlers();
 
     /**