fix(graphql): gql relationship has many null values

[6TELOIV]

What?

The gql api responds with a null value error when running a query on a hasMany relationship field where the related collection has read access control blocking some of the records from being read.

Why?

The cause of that issue is that the payload gql relationship resolver function must return new GraphQLList(new GraphQLNonNull(type)) but the context.req.payloadDataLoader.load will return null for data that is blocked by access control. This results in the resolver function returning null values, and causes the error.

How?

I fixed this by adding a filter to remove the null values from the result: return results.filter((result) => result !== null). I believe this is the appropriate way to fix this as explained below.

Because the code is written to run the data loading in parallel, the n promises all run and insert something into the results array at their predetermined index to keep the items in order. Thinking about trying to do the same while skipping null values, I would think you'd need to use some sort of heap/sorted data structure, which would slow the insertion for the n threads to something below-constant. This makes the end result something greater than O(n). Compare that to this proposed result, which retains the existing O(n) time to run the threads, and adds O(n) more work to filter out the null values, resulting in O(n)+O(n)=O(n) execution time.

Fixes #9236

[6TELOIV]

This PR also adds a regression test for the issue which I used to verify the presence of the issue as well as the fix. If you checkout commit #3399b29 you can see the failing test.