`useDefault: true` doesn't work when a property is initialized to `null`

Which package(s) are affected?

Lit Core (lit / lit-html / lit-element / reactive-element)

Description

The docs for useDefault say a property must be initialized, but don't say the initial value can't be null or undefined and it can't: useDefault uses ?? to both record and restore the default value, so it can't tell "no default recorded" from "the default is null/undefined".

This causes bugs in a few cases:

• null initial value with standard decorators throws when the element is constructed.
• foo: string | undefined = undefined: removing the attribute sets the property to null rather than undefined.
• accessor foo: string | undefined = undefined, or a getter/setter pair: the default is never recorded at initialization (the init paths skip undefined values), so the first value the user sets is recorded as the "default". Removing the attribute then restores that value instead of undefined, i.e. it appears to do nothing.

Reproduction

The null-default case:

class MyEl extends LitElement {
  @property({reflect: true, useDefault: true})
  accessor foo: string | null = null;
}

new MyEl();
// TypeError: Private element is not present on this object
//   at ReactiveElement._$changeProperty (reactive-element.ts:1339)
//   at init (decorators/property.ts:157)

Workaround

Don't use null defaults?

Is this a regression?

No or unsure. This never worked, or I haven't tried before.

Affected versions

3.3

Browser/OS/Node environment

n/a