Add 'fallback' image when there is no 'featured image'

Hello there,

There are some ways to set the fallback image for an image field in the admin column.

- Create a custom admin column: please read more on the documentation https://docs.metabox.io/extensions/mb-admin-columns/#creating-custom-class

- Use the filter hook rwmb_the_value to modify the field value: please read more on the documentation
https://docs.metabox.io/filters/rwmb-the-value/
and this topic
https://support.metabox.io/topic/icons-instead-of-values-to-indicate-a-confirmed-account-as-a-user-admin-column/#post-36979