Avoid extra query in tax calculation
Avoid extra query in tax calculation

When Spree::Zone.default_tax is nil, the TaxHelpers module ends up
calling Spree::TaxRate.for_zone(nil). Previously this would generate a
query with `WHERE (0=1)`, instead we can just use `AR::Base.none` and
avoid the query entirely.

pervino/solidus,Arpsara/solidus,Arpsara/solidus,Arpsara/solidus,pervino/solidus,Arpsara/solidus,pervino/solidus,pervino/solidus