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

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"
}

Reproduce the issue in the Designer by setting a TextBlock's fontType to Monospace and then back to “(not set)”. Trace how the Designer updates and serializes the Card JSON, then verify that resetting the attribute removes the fontType property and that the behavior is covered by an appropriate regression test.