[C++][Dataset] Refactor Partitioning to explicitly treat null and absent fields identically

ARROW-10438 adds support for partition expressions with explicit absence of a partition key by including an is_null(field_ref("absent key field name")) in the conjunction. Whenever possible, this should be preferred to an equivalent conjunction which simply omits an equality expression for the missing field.

Additionally since an absent partition key and a null partition key is semantically equivalent to a null valued partition key, we should ensure there is no difference in behavior. Currently, equal(field_ref("a"), literal(0)) and and_(equal(field_ref("a"), literal(0)), is_null("b")) are formatted differently