Throw Error when referencing uninit typed prop in __sleep

Throw Error when referencing uninit typed prop in __sleep

Previously this generated a notice, but would likely generate an
Error when unserializing.

Now we treat it with the same distinction as direct property
accesses, i.e. referencing an unset/undefined normal property
stays a notice, while a typed property becomes an Error exception.

This fixed bug #79002.

Closes GH-5050.