Spree::Variant could not be consistent

[kennyadsl]

Describe the bug

Right now we can have a Spree::Variant instance that is not consistent since the price cannot be created without a product associated with the variant.

When [set_price]
(

solidus/core/app/models/spree/variant.rb

Line 75 in 8c8986a

before_validation :set_price

) is called before validation it raises an error if the product is not set.

Solidus Version:

master

To Reproduce

Steps to reproduce the behavior:

1. Open a rails console on a sandbox generated on master
2. Run:

v = Spree::Variant.new
=> #<Spree::Variant id: nil, sku: "", weight: 0.0, height: nil, width: nil, depth: nil, deleted_at: nil, is_master: false, product_id: nil, cost_price: nil, position: nil, cost_currency: nil, track_inventory: true, tax_category_id: nil, updated_at: nil, created_at: nil>
irb(main):090:0> v.valid?
Traceback (most recent call last):
        1: from (irb):90
RuntimeError (No master variant found to infer price)

Expected behavior

It should not raise an exception and an error should be added to the object instead.