Ignore Exchequer defaultwarehouse location if Multi Locations is disabled

The Exchequer integration will import a 'Total' warehouse of --- for both Warehouses and Stock by Warehouse (stock levels) sections if the Exchequer 'Multi Locations' system setting is disabled.

The Product Item import does not check this setting, so if a stock record in Exchequer has a value in the Location field, and 'Multi Locations' is disabled, we will not be able to import/update the product (as it tries to populate DefaultWarehouse with a location that we don't have in our Warehouses table).