[~BUGFIX] Fixed notice "undefined _attribute_set" during import if developer mode is on

We got a PHP notice that _attribute_set is not defined if import contains at least one category.
The occures only if developer mode is on, display_errors = 1 and product is new.

Example CSV file:

```
sku,_store,_type,_attribute_set,name,price,description,short_description,weight,tax_class_id,status,visibility,qty,color,_root_category,_category,media_gallery
sku0,default,simple,Default,"Standard name",1.99,"Standard desc","Standard short desc",0,2,1,4,1999,,,,703
,,,,,,,,,,,,,,"Root Catalog",Furniture

I added an additional check with isset. It's the same like some lines before for `COL_TYPE`.