Right now, when empty values are passed as quantity in the API call
that creates line items, the line item is populated with quantity 0.

This is not the expected behavior, as it makes no sense to have a line
item with 0 as quantity.

Still, if someone wants to force the value to 0, this is still possible
but 0 needs to be explicitly passed as quantity value. A spec has been
added for this scenario as well.

Its spec was not testing single-level nested_attributes, which in the
codebase occurs in the Api::LineItemsController [1].

[1] https://github.com/solidusio/solidus/blob/df68c0c748ded6a595d28c37a9460126c6d12ecb/api/app/controllers/spree/api/line_items_controller.rb#L56-L62

This method allows the deletion of LineItems where the quantity is set
to 0 or less. This is possible because LineItem allows a quantity of 0
on save, but soon this will be disallowed, so we have to manually
destroy these LineItems and remove them from the params before saving.