Hi,
As already pointed by Colin, I see as well requests every hour with an empty sku value.
If I set in "Edit Variant" form my fulfillment service in "Fulfillment" and "Inventory policy" settings, as soon as I press 'Save' Shopify makes a call to my callback (fetch_stock.json) passing the right sku in the query string!
It seems it is only with the auto requests that happens every hour that my callback is called with an empty sku (shop & timestamp have a value, sku has an empty value). I hope this info can help to address this issue.
Thanks.