distinguish between null and empty map in API (#10829)

patches/api/General-ItemMeta-fixes.patch

@@ -71,3 +71,19 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
      /**
       * Checks to see if this item has a maximum amount of damage.
       *
+diff --git a/src/main/java/org/bukkit/inventory/meta/ItemMeta.java b/src/main/java/org/bukkit/inventory/meta/ItemMeta.java
+index 0000000000000000000000000000000000000000..0000000000000000000000000000000000000000 100644
+--- a/src/main/java/org/bukkit/inventory/meta/ItemMeta.java
++++ b/src/main/java/org/bukkit/inventory/meta/ItemMeta.java
+@@ -0,0 +0,0 @@ public interface ItemMeta extends Cloneable, ConfigurationSerializable, Persiste
+ 
+     /**
+      * Set all {@link Attribute}s and their {@link AttributeModifier}s.
+-     * To clear all currently set Attributes and AttributeModifiers use
+-     * null or an empty Multimap.
++     * To clear all custom attribute modifiers, use {@code null}. To set
++     * no modifiers (which will override the default modifiers), use an
++     * empty map.
+      * If not null nor empty, this will filter all entries that are not-null
+      * and add them to the ItemStack.
+      *

patches/server/General-ItemMeta-fixes.patch

@@ -954,7 +954,17 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
          return this.attributeModifiers.put(attribute, modifier);
      }
  
-@@ -0,0 +0,0 @@ class CraftMetaItem implements ItemMeta, Damageable, Repairable, BlockDataMeta {
+     @Override
+     public void setAttributeModifiers(@Nullable Multimap<Attribute, AttributeModifier> attributeModifiers) {
+-        if (attributeModifiers == null || attributeModifiers.isEmpty()) {
++        // Paper start - distinguish between null and empty
++        if (attributeModifiers == null) {
++            this.attributeModifiers = null;
++            return;
++        }
++        if (attributeModifiers.isEmpty()) {
++            // Paper end - distinguish between null and empty
+             this.attributeModifiers = LinkedHashMultimap.create();
              return;
          }