Fix migration to support legacy stores with deleted variants and line items

Trying to migrate my existing store and the migration for adding line item ids to inventory units fails when a variant has since been deleted or when a line item no longer exists.

Variants are not found as the Spree::Shipment includes scope will not find deleted variants. Removing the scope will find them. Here is some console output from testing locally:

**With variant include**
```ruby
pry(main)> Spree::Shipment.includes({ :inventory_units => :variant }, :order).find(...).inventory_units.group_by(&:variant).each { |v, u| puts v.nil? }
  Spree::Shipment Load (0.4ms)  SELECT "spree_shipments".* FROM "spree_shipments" WHERE "spree_shipments"."id" = $1 LIMIT 1  [["id", 2178]]
  Spree::InventoryUnit Load (0.3ms)  SELECT "spree_inventory_units".* FROM "spree_inventory_units" WHERE "spree_inventory_units"."shipment_id" IN (2178)
  Spree::Variant Load (0.3ms)  SELECT "spree_variants".* FROM "spree_variants" WHERE "spree_variants"."id" IN (4793, 754, 4840, 1251, 378, 2719, 4739, 2340) AND ("spree_variants".deleted_at IS NULL)
  Spree::Order Load (0.2ms)  SELECT "spree_orders".* FROM "spree_orders" WHERE "spree_orders"."id" IN (5636)
false
false
false
false
false
false
true <-------- Nil variant here
false
```

**Without variant include**
```ruby
pry(main)> Spree::Shipment.includes(:inventory_units, :order).find(...).inventory_units.group_by(&:variant).each { |v, u| puts v.nil? }
  Spree::Shipment Load (0.4ms)  SELECT "spree_shipments".* FROM "spree_shipments" WHERE "spree_shipments"."id" = $1 LIMIT 1  [["id", 2178]]
  Spree::InventoryUnit Load (0.3ms)  SELECT "spree_inventory_units".* FROM "spree_inventory_units" WHERE "spree_inventory_units"."shipment_id" IN (2178)
  Spree::Order Load (0.2ms)  SELECT "spree_orders".* FROM "spree_orders" WHERE "spree_orders"."id" IN (5636)
  Spree::Variant Load (0.7ms)  SELECT "spree_variants".* FROM "spree_variants" WHERE "spree_variants"."id" = $1 ORDER BY "spree_variants"."id" ASC LIMIT 1  [["id", 4793]]
  Spree::Variant Load (0.2ms)  SELECT "spree_variants".* FROM "spree_variants" WHERE "spree_variants"."id" = $1 ORDER BY "spree_variants"."id" ASC LIMIT 1  [["id", 754]]
  Spree::Variant Load (0.2ms)  SELECT "spree_variants".* FROM "spree_variants" WHERE "spree_variants"."id" = $1 ORDER BY "spree_variants"."id" ASC LIMIT 1  [["id", 4840]]
  Spree::Variant Load (0.2ms)  SELECT "spree_variants".* FROM "spree_variants" WHERE "spree_variants"."id" = $1 ORDER BY "spree_variants"."id" ASC LIMIT 1  [["id", 1251]]
  Spree::Variant Load (0.2ms)  SELECT "spree_variants".* FROM "spree_variants" WHERE "spree_variants"."id" = $1 ORDER BY "spree_variants"."id" ASC LIMIT 1  [["id", 1251]]
  Spree::Variant Load (0.2ms)  SELECT "spree_variants".* FROM "spree_variants" WHERE "spree_variants"."id" = $1 ORDER BY "spree_variants"."id" ASC LIMIT 1  [["id", 1251]]
  Spree::Variant Load (0.2ms)  SELECT "spree_variants".* FROM "spree_variants" WHERE "spree_variants"."id" = $1 ORDER BY "spree_variants"."id" ASC LIMIT 1  [["id", 1251]]
  Spree::Variant Load (0.1ms)  SELECT "spree_variants".* FROM "spree_variants" WHERE "spree_variants"."id" = $1 ORDER BY "spree_variants"."id" ASC LIMIT 1  [["id", 1251]]
  Spree::Variant Load (0.2ms)  SELECT "spree_variants".* FROM "spree_variants" WHERE "spree_variants"."id" = $1 ORDER BY "spree_variants"."id" ASC LIMIT 1  [["id", 1251]]
  Spree::Variant Load (0.2ms)  SELECT "spree_variants".* FROM "spree_variants" WHERE "spree_variants"."id" = $1 ORDER BY "spree_variants"."id" ASC LIMIT 1  [["id", 378]]
  Spree::Variant Load (0.2ms)  SELECT "spree_variants".* FROM "spree_variants" WHERE "spree_variants"."id" = $1 ORDER BY "spree_variants"."id" ASC LIMIT 1  [["id", 2719]]
  Spree::Variant Load (0.1ms)  SELECT "spree_variants".* FROM "spree_variants" WHERE "spree_variants"."id" = $1 ORDER BY "spree_variants"."id" ASC LIMIT 1  [["id", 4739]]
  Spree::Variant Load (0.2ms)  SELECT "spree_variants".* FROM "spree_variants" WHERE "spree_variants"."id" = $1 ORDER BY "spree_variants"."id" ASC LIMIT 1  [["id", 2340]]
false
false
false
false
false
false
false
false
```

I have not found any way other than skipping update where the corresponding line item no longer exists.