Fix bug #79447

Fix bug #79447

Partially reverts 846b6479537a112d1ded725e6484e46462048b35: instead of
throwing, this skips uninitialized typed properties when serializing objects.

This makes serialize with __sleep() behave the same as serialize()
without __sleep().

As in the non-__sleep() case, unserialize(serialize($x)) identity
may not be preserved due to replacement of uninitialized/unset
properties with default values. Fixing this will require changes to
the serialization format.

Closes GH-5396.