fix(graphql): 500 error when querying hasMany field data that is filtered by access-control

[ikenox]

This PR closes #6518. Just filtering null item from the results array makes thing work well.

Description

• [x] I have read and understand the CONTRIBUTING.md document in this repository.

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] I have added tests that prove my fix is effective or that my feature works
• [x] Existing test suite passes locally with my changes