core: add `Sequence::get_or_default` and `get_or_null`
core: add `Sequence::get_or_default` and `get_or_null`

Signed-off-by: Jean Privat <51f8b1fa9b424745378826727452997ee2a7c3d7@pryen.org>

privat/nit,privat/nit,privat/nit,privat/nit,privat/nit,privat/nit,privat/nit,privat/nit,privat/nit,privat/nit