Um there is something not quite right in your code base earlier on if it's inserting empty arrays.
You need to find the code that is inserting into this table and check if there is an empty array being created and if there is, pass through an empty string or nothing if it's not a required field.
However if you want to select them using Eloquent you would do:
$items = Item::where('sku_purchase_price', '\[\]')->get();
But yes, don't do this, find the actual problem in your code first rather than trying to select empty arrays from the db...