SPIGOT-5246: PlayerQuitEvent.get/setQuitMessage() is incorrectly marked as NotNull

By: md_5 <git@md-5.net>

paper-api/src/main/java/org/bukkit/event/player/PlayerQuitEvent.java

@@ -3,6 +3,7 @@ package org.bukkit.event.player;
 import org.bukkit.entity.Player;
 import org.bukkit.event.HandlerList;
 import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 /**
  * Called when a player leaves a server
@@ -11,7 +12,7 @@ public class PlayerQuitEvent extends PlayerEvent {
     private static final HandlerList handlers = new HandlerList();
     private String quitMessage;
 
-    public PlayerQuitEvent(@NotNull final Player who, @NotNull final String quitMessage) {
+    public PlayerQuitEvent(@NotNull final Player who, @Nullable final String quitMessage) {
         super(who);
         this.quitMessage = quitMessage;
     }
@@ -21,7 +22,7 @@ public class PlayerQuitEvent extends PlayerEvent {
      *
      * @return string quit message
      */
-    @NotNull
+    @Nullable
     public String getQuitMessage() {
         return quitMessage;
     }
@@ -31,7 +32,7 @@ public class PlayerQuitEvent extends PlayerEvent {
      *
      * @param quitMessage quit message
      */
-    public void setQuitMessage(@NotNull String quitMessage) {
+    public void setQuitMessage(@Nullable String quitMessage) {
         this.quitMessage = quitMessage;
     }