Hello,
We do have a default option for shop banners:
wp admin > woocommerce > wc vendors > pro (scroll to the bottom). You can choose a default image for the shop banners so it is not just blank.
If that is not working, then you will need to discuss with BuddyBoss how you can either show a default banner, or perhaps have the shop banner region measure differently if there is no banner uploaded by the vendor.
