Header image unable to revert to default image

[tw77]

Description

If you upload and set a new header image for a network, after publishing changes, it isn't possible to revert to the default header image. However, it is possible to switch to other uploaded header images. (A workaround is to re-upload the default header image file and set it as a new header image.) This was reported on https://csi.pressbooks.pub/ and can be replicated on Integrations.

Steps to Reproduce

1. From the Customize menu, add a new header image.
2. Select it as the header image for the network.
3. Once selected, the new header image appears onscreen.
4. Press Publish to save changes.
5. Navigate to the network home page, confirming that the new header image is in place.
6. From the Customize menu, select the default header image as the header image for the network.
7. Once selected, the default header image appears onscreen.
8. Press Publish to save changes.
9. Navigate to the network home page.

Expected behavior: The change back to the default header image has been saved, updating the network header image.

Actual behavior: The change back to the default header image has not been saved. This is the case even when viewing in a different browser / incognito window and after waiting for a period of time for the change to take effect. Returning to the Customize menu, there also isn't a trace of the attempted change.