handle recent ItemMeta changes in serialization logic

patches/server/1044-General-ItemMeta-fixes.patch

@@ -260,7 +260,7 @@ index 8e0dd4b7a7a25a8beb27b507047bc48d8227627c..cf5d27ccc2225bac3aa57912f444f95d
  
      @Override
 diff --git a/src/main/java/org/bukkit/craftbukkit/inventory/CraftMetaItem.java b/src/main/java/org/bukkit/craftbukkit/inventory/CraftMetaItem.java
-index 0bc100430483f88bc7edf17645250b2862ccc1d5..c81d8e09c6f6ac4cec9cd57e9af41a9a60b24bb1 100644
+index 0bc100430483f88bc7edf17645250b2862ccc1d5..db3e74f22f555e3bb5b304af303cdd46a5377894 100644
 --- a/src/main/java/org/bukkit/craftbukkit/inventory/CraftMetaItem.java
 +++ b/src/main/java/org/bukkit/craftbukkit/inventory/CraftMetaItem.java
 @@ -165,9 +165,10 @@ class CraftMetaItem implements ItemMeta, Damageable, Repairable, BlockDataMeta {
@@ -296,6 +296,15 @@ index 0bc100430483f88bc7edf17645250b2862ccc1d5..c81d8e09c6f6ac4cec9cd57e9af41a9a
          CraftMetaItem.getOrEmpty(tag, CraftMetaItem.NAME).ifPresent((component) -> {
              this.displayName = component;
          });
+@@ -725,7 +731,7 @@ class CraftMetaItem implements ItemMeta, Damageable, Repairable, BlockDataMeta {
+         Map<?, ?> mods = SerializableMeta.getObject(Map.class, map, key.BUKKIT, true);
+         Multimap<Attribute, AttributeModifier> result = LinkedHashMultimap.create();
+         if (mods == null) {
+-            return result;
++            return null; // Paper - null is different from an empty map
+         }
+ 
+         for (Object obj : mods.keySet()) {
 @@ -879,10 +885,8 @@ class CraftMetaItem implements ItemMeta, Damageable, Repairable, BlockDataMeta {
      }
  
@@ -386,6 +395,15 @@ index 0bc100430483f88bc7edf17645250b2862ccc1d5..c81d8e09c6f6ac4cec9cd57e9af41a9a
                  clone.attributeModifiers = LinkedHashMultimap.create(this.attributeModifiers);
              }
              if (this.customTag != null) {
+@@ -1787,7 +1795,7 @@ class CraftMetaItem implements ItemMeta, Damageable, Repairable, BlockDataMeta {
+     }
+ 
+     static void serializeModifiers(Multimap<Attribute, AttributeModifier> modifiers, ImmutableMap.Builder<String, Object> builder, ItemMetaKey key) {
+-        if (modifiers == null || modifiers.isEmpty()) {
++        if (modifiers == null/* || modifiers.isEmpty()*/) { // Paper - null and an empty map have different behaviors
+             return;
+         }
+ 
 @@ -1869,7 +1877,7 @@ class CraftMetaItem implements ItemMeta, Damageable, Repairable, BlockDataMeta {
      // Paper start - improve checking handled tags
      @org.jetbrains.annotations.VisibleForTesting