12973: fix error raised in variant creation

- if the sheet doesn't have the units present, then the variant is not saved due to model validation
- After that, while assigning on_hand value, error is raised that the variant is not created first
- Now this commit makes sure that the variant is created before implementing above logic

app/models/product_import/entry_validator.rb

@@ -79,10 +79,9 @@ module ProductImport
         if entry.attributes['on_hand'].present?
           new_variant.on_hand = entry.attributes['on_hand']
         end
+        check_on_hand_nil(entry, new_variant)
       end
 
-      check_on_hand_nil(entry, new_variant)
-
       if new_variant.valid?
         entry.product_object = new_variant
         entry.validates_as = 'new_variant' unless entry.errors?