Fix ModalMapping::getAsAttachmentList if no attachments were submitted (#2941)

Author: archer-321

src/main/java/net/dv8tion/jda/api/interactions/modals/ModalMapping.java

@@ -29,6 +29,7 @@
 import net.dv8tion.jda.internal.utils.Helpers;
 
 import javax.annotation.Nonnull;
+import java.util.Collections;
 import java.util.List;
 import java.util.Objects;
 
@@ -203,6 +204,9 @@ public List<Message.Attachment> getAsAttachmentList()
         if (type != Component.Type.FILE_UPLOAD)
             typeError("List<Message.Attachment>");
 
+        if (resolved.isNull("attachments"))
+            return Collections.emptyList();
+
         final DataObject attachments = resolved.getObject("attachments");
         final EntityBuilder entityBuilder = interaction.getJDA().getEntityBuilder();
         return value.getArray("values")