Fix/variant optional attributes (#6300)

* Force remount of the form when variantid changes to reset its state

* Add changeset

---------

Co-authored-by: Lukasz Ostrowski <lukasz.ostrowski@saleor.io>

Author: mirekm

.changeset/easy-beans-notice.md

@@ -0,0 +1,5 @@
+---
+"saleor-dashboard": patch
+---
+
+Fix variant form initialisation

src/products/components/ProductVariantPage/ProductVariantPage.tsx

@@ -246,6 +246,7 @@ export const ProductVariantPage = ({
       </TopNav>
       <DetailPageLayout.Content>
         <ProductVariantUpdateForm
+          key={variant?.id}
           variant={variant}
           onSubmit={onSubmit}
           currentChannels={channels}