special casing fallback behavior when attribute is set to null will make it more difficult to tool against

```
Note that a value of null satisfies the condition of having the attribute. A value of null
has the usual meaning where null is an acceptable value for the attribute; otherwise, a value
of null shall be treated as the attribute being unset (and shall suppress falling back to the
non-configuration-specific value).
```

In JSON parsers, where you specify a structure of types and the parser tries to fill out those types, typically an unset attribute and an attribute set to null both results in a None value in an optional. In cases where you must tell the difference between an unset value and a null value, you'll end up with a nested option type. This makes it much more challenging to tool around this spec.

It would be much easier if `tag: null` meant the same as the tag not being specified. If you want to unset the values and not allow a fallback, set the value to an empty string or array. For example `tag: []` or `tag: ""` would cause an overriding to an empty value.