closes#9085
Fixes an issue, where the client sets image properties to `""` after deleting the image. This causes problems with the query filter (see https://github.com/TryGhost/GQL/issues/24), as they have to be `null`.
Added a check in the model layer saving method to set value to `null`, when the property is empty.
Affected models and properties:
- `posts`:
- `feature_image`
- `og_image`
- `twitter_image`
- `users`:
- `profile_image`
- `cover_image`
- `tags`:
- `feature_image`