Fix Variant Details console errors (#742)

* Add default empty unit when there is no specified one

* Add index required in draggable tables

* Select fields should get undefined value instead of null

src/products/components/ProductShipping/ProductShipping.tsx

@@ -49,7 +49,9 @@ const ProductShipping: React.FC<ProductShippingProps> = props => {
             onChange={onChange}
             InputProps={{
               endAdornment: (
-                <InputAdornment position="end">{weightUnit}</InputAdornment>
+                <InputAdornment position="end">
+                  {weightUnit || ""}
+                </InputAdornment>
               ),
               inputProps: {
                 min: 0

src/products/components/ProductVariantAttributes/ProductVariantAttributes.tsx

@@ -52,7 +52,7 @@ function getAttributeValue(
   attributes: VariantAttributeInput[]
 ): string {
   const attribute = attributes.find(attr => attr.id === id);
-  return attribute.value;
+  return attribute?.value === null ? undefined : attribute.value;
 }
 
 function getAttributeValueChoices(

src/products/components/ProductVariantNavigation/ProductVariantNavigation.tsx

@@ -90,7 +90,7 @@ const ProductVariantNavigation: React.FC<ProductVariantNavigationProps> = props
             <SortableTableRow
               hover={!!variant}
               key={variant ? variant.id : "skeleton"}
-              index={variantIndex}
+              index={variantIndex || 0}
               className={classNames(classes.link, {
                 [classes.tabActive]: variant && variant.id === current
               })}