HSM Template: Adding fallback image

[akanshaaa19]

Describe the bug
On editing a hsm template with type as IMAGE, if there is no media returned we should show a fallback image.

To Reproduce
Steps to reproduce the behavior:

1. Go to HSM templates and find a media template
2. Click on Edit
3. There is no image shown in the simulator

[kurund]

@akanshaaa19

As discussed we can't save only the attachment type without the attachment URL. Can you please check if this is something we can handle in the backend?

Showing fallback images seems wrong to me.