Fix spree product touch supplier

Take into account import product scenario, in this case the variant
doesn't have an assigned supplier yet.

app/models/spree/product.rb

@@ -77,6 +77,7 @@ module Spree
     after_update :touch_supplier, if: :saved_change_to_primary_taxon_id?
     around_destroy :destruction
     after_save :update_units
+    after_touch :touch_supplier
 
     # -- Scopes
     scope :with_properties, ->(*property_ids) {
@@ -333,7 +334,12 @@ module Spree
 
       # Assume the product supplier is the supplier of the first variant
       # Will breack if product has mutiple variants with different supplier
-      variants.first.supplier.touch
+      first_variant = variants.first
+
+      # The variant is invalid if no supplier is present, but this method can be triggered when
+      # importing product. In this scenario the variant has not been updated with the supplier yet
+      # hence the check.
+      first_variant.supplier.touch if first_variant.supplier.present?
     end
 
     def touch_distributors

spec/models/spree/product_spec.rb

@@ -334,6 +334,14 @@ module Spree
           expect { product.touch }
             .to change { supplier.reload.updated_at }
         end
+
+        context "when the first variant is missing supplier" do
+          it "doesn't blow up" do
+            product.variants.first.update_attribute(:supplier_id, nil)
+
+            expect { product.touch }.to_not raise_error
+          end
+        end
       end
 
       it "updates units when saved change to variant unit" do