Add check for non-empty sku list on grouped product import

Currently it is not possible to import grouped products without any associated simple products. It will result in a database error since the plugin tries to autoLink simple products which may or may not succeed (depending on the skus).

It is necessary to add a check in fixedLink method if the $skulist is non-empty. Else there will always be a database error when working with an empty list, resulting in the failure of the import of that grouped product.