Tiled Gallery Block: If a image is deleted from the Media Library, it cannot be removed from the block

### Steps to reproduce the behavior

1. Create a Tiled Gallery block with multiple images
2. Go to the Media Library and delete one of the images that are present in the Tiled Gallery
3. Clear cache
4. Notice the gallery has an empty space
5. Try to remove the empty space

#### What I expected to happen

To be able to remove the deleted image, same as other images. Or for the deleted image to be automatically removed from the Tiled Gallery

#### What actually happened

The gallery displays an empty space with no option to remove the image. If the gallery has lots of images the only option for a user to remove the empty space would be to recreate the gallery

### Context

User report: 4086203-zd-woothemes

#### Browser / OS version
macOS Big Sur 11.4 (20F71)
Chrome Version 91.0.4472.114 (Official Build) (x86_64)

#### Is this specific to the applied theme? Which one?
No theme specific

#### Does this happen on simple or atomic sites or both?
Tested on a simple site

#### Is there any console output or error text?
No, only the 404 error for the deleted image.

#### Level of impact (Does it block purchases? Does it affect more than just one site?)
Affects any site that uses the Tiled Gallery block

#### Reproducibility (Consistent, Intermittent) Leave empty for consistent.

#### Screenshot / Video: If applicable, add screenshots to help explain your problem.

https://user-images.githubusercontent.com/3696121/123173323-0fa0e780-d444-11eb-8341-af78a3f50225.mp4

Start by reproducing the issue in the Tiled Gallery block after deleting an image from the Media Library and clearing the cache. Trace how the block represents the deleted image, then verify that the empty item can be removed or is automatically removed without requiring the gallery to be recreated.

Written by the indexing model from the issue text.