The object_count of that query is 0, which means that it's empty. With no data to provide, we would default to "null" as a returned URL.
For your other 41 jobs, is the object count for all of them "500", or is there one that has an object count lower than 500? In which case, it would make sense why your last job has 0 data since one of the jobs isn't completely full.
Here are the results of all 42 jobs. In which, there are 17 Jobs that return Null that actually want to have data
May I ask if Shopify has a block mechanism when there are too many requests continuously when using BulkOperation?
(because from the following request all responses that Shopify returns are NULL)