fix(model): remove empty list initialization from CarouselBlock.elements

Follow project convention of not initializing List fields to empty
collections — null vs empty has semantic meaning in the Slack API.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: srtaalej

slack-api-model/src/main/java/com/slack/api/model/block/CarouselBlock.java

@@ -5,7 +5,6 @@
 import lombok.Data;
 import lombok.NoArgsConstructor;
 
-import java.util.ArrayList;
 import java.util.List;
 
 /**
@@ -25,8 +24,7 @@ public class CarouselBlock implements LayoutBlock {
     /**
      * An array of {@link CardBlock card} blocks. Must contain between 1 and 10 cards.
      */
-    @Builder.Default
-    private List<CardBlock> elements = new ArrayList<>();
+    private List<CardBlock> elements;
 
     private String blockId;
 }