unfocusedAppearance ignores backgroundImage set to null and continued to display image

### Windows Terminal version

1.13.10395.0

### Windows build number

10.0.22567.1

### Other Software

_No response_

### Steps to reproduce

It is possible to set in the UI, and via the settings XML, for there to be no background image when a terminal windows is unfocused using the example below:
"unfocusedAppearance":
{
"backgroundImage": null
}

### Expected Behavior

The expected behaviour is that the background image will disappear when set to null or change image to a new image if a value is set

### Actual Behavior

Actual behaviour is that the background image continues to be displayed if set to null, although a new image is displayed if a correct file location is set.

Start by reproducing the issue in Windows Terminal 1.13.10395.0 with unfocusedAppearance.backgroundImage set to null, then trace the handling of that setting. Done means the image disappears when the value is null, while assigning a valid new image still displays the replacement.

Written by the indexing model from the issue text.