Consider nil..nil range as nil

[DanAndreasson]

Currently range: nil..nil always return zero results as the query includes WHERE ("users"."created_at" >= NULL). This PR changes so that a nil..nil range is considered as "without begin and without end".

It makes it easier to compose queries such as

User.group_by_year(:created_at, range: starts_at..ends_at).count

where starts_at and ends_at is both optional

[ankane]

Looks great (and is the same behavior as Active Record's where). Thanks @DanAndreasson!