[Designer] Setting attributes to (not set) does not actually remove the property

[sabberworm]

Problem Description

When using the designer to set an attribute, for example fontType to “Monospace”, the corresponding JSON property is added:

However, when later changing the attribute back to “(not set)”, I expect the JSON property to be removed. However, it is kept as-is.

Card JSON

{
    "type": "AdaptiveCard",
    "body": [
        {
            "type": "TextBlock",
            "text": "${title}",
            "fontType": "Monospace"
        }
    ],
    "$schema": "http://adaptivecards.io/schemas/adaptive-card.json",
    "version": "1.6"
}