PIM-3311: fix creation of products with missing identifier during imports

The impact of this bug is less important than predicted. Only product imports are impacted.

Guys, I know this fix is quite dirty, due to the following reasons : 
 * product identifier are not validated directly via the product object (like all other entities) as sku are product values
 * the SKU product value is not validated because SKU does not appear in the `$transformedColumns` of `ProductTransformer` (that's the real bug). As I can not determine why at the moment, this fix is a workaround
 * instead of adding an error, I would have liked to launch a `MissingIdentifierException`. But it's impossible at the moment, because the product would not be considered as skipped.