If you won’t want the empty box to be displayed, then you need to set a default fallback image when no featured image is found. This can also be very useful when you forget to set a featured image for your post.
In this article, I will show you how to add a default featured image in WordPress using a plugin.
First thing you need to do is install and activate the Default Featured Image plugin.
Once installed and activated, the plugin adds an option under Settings » Media to choose a default image.
Upload your default image and save settings. This image could be any image, for example an image of your site logo on a plain background.
Now, when your theme looks for a featured image and does not find one, it will display the default image.
If you would like to set a fallback Featured Image based on post category, using this same plugin, read the conditional featured images tutorial.
I hope this helps.