Fix PlayerSwapHandItemsEvent NPE when a hand set to null (#9763)

* Fix PlayerSwapHandItemsEvent throwing exception when mainhand or offhand set to null

* use fully qualified import

* Use ItemStack#empty() instead of new ItemStack

* Add NotNull annotation to getters

* Add missing Paper comments

patches/api/0441-Fix-PlayerSwapHandItemsEvent-throwing-exception-when.patch

@@ -0,0 +1,47 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Tamion <70228790+notTamion@users.noreply.github.com>
+Date: Mon, 25 Sep 2023 19:55:51 +0200
+Subject: [PATCH] Fix PlayerSwapHandItemsEvent throwing exception when mainhand
+ or offhand set to null
+
+
+diff --git a/src/main/java/org/bukkit/event/player/PlayerSwapHandItemsEvent.java b/src/main/java/org/bukkit/event/player/PlayerSwapHandItemsEvent.java
+index 9f592317c920589c22a5fb8e916c6ca58ebe5c59..39dd08de71b8b52fe3462c105ecdbfc1cd2cd9a3 100644
+--- a/src/main/java/org/bukkit/event/player/PlayerSwapHandItemsEvent.java
++++ b/src/main/java/org/bukkit/event/player/PlayerSwapHandItemsEvent.java
+@@ -31,7 +31,7 @@ public class PlayerSwapHandItemsEvent extends PlayerEvent implements Cancellable
+      *
+      * @return item in the main hand
+      */
+-    @Nullable
++    @NotNull // Paper
+     public ItemStack getMainHandItem() {
+         return mainHandItem;
+     }
+@@ -42,7 +42,7 @@ public class PlayerSwapHandItemsEvent extends PlayerEvent implements Cancellable
+      * @param mainHandItem new item in the main hand
+      */
+     public void setMainHandItem(@Nullable ItemStack mainHandItem) {
+-        this.mainHandItem = mainHandItem;
++        this.mainHandItem = mainHandItem == null ? ItemStack.empty() : mainHandItem; // Paper
+     }
+ 
+     /**
+@@ -50,7 +50,7 @@ public class PlayerSwapHandItemsEvent extends PlayerEvent implements Cancellable
+      *
+      * @return item in the off hand
+      */
+-    @Nullable
++    @NotNull // Paper
+     public ItemStack getOffHandItem() {
+         return offHandItem;
+     }
+@@ -61,7 +61,7 @@ public class PlayerSwapHandItemsEvent extends PlayerEvent implements Cancellable
+      * @param offHandItem new item in the off hand
+      */
+     public void setOffHandItem(@Nullable ItemStack offHandItem) {
+-        this.offHandItem = offHandItem;
++        this.offHandItem = offHandItem == null ? ItemStack.empty() : offHandItem; // Paper
+     }
+ 
+     @Override