fix Option<~ZeroSizeType>

1778b6361627c5894bf75ffecf427573af02d390 provided the guarantee of no
`exchange_free` calls for ~ZeroSizeType, so a sentinel can now be used
without overhead.

Closes #11998
auto merge of #13267 : thestinger/rust/zero_size_non_null, r=pnkfelix

1778b6361627c5894bf75ffecf427573af02d390 provided the guarantee of no
`exchange_free` calls for ~ZeroSizeType, so a sentinel can now be used
without overhead.

Closes #11998