Starlight - Gray bar visible even when no caption is used with carousel widget

Summary: We added a gray transparent bar to help with caption text visibility in carousel widget. However, when no caption is used, the transparent bar should not be overlaid on the image.

Steps to Reproduce: You can see the issue here (very bottom of page): https://spotlight-stage.library.ucsb.edu/starlight/documentation/feature/item-carousel

The top version of the widget has a caption, the bottom version doesn't.

Expected Results: When no caption is used with the carousel widget, the text background bar should not be added.

Actual Results: The text background bar is added regardless of whether a caption used with the widget.

Notes: