fix: collection might contain null values when eager loading

[SychO9]

Changes proposed in this pull request: Plucking seems to include null values as well, so we need to filter the collection.

Confirmed

• [ ] Frontend changes: tested on a local Flarum installation.
• [ ] Backend changes: tests are green (run composer test).

Required changes:

• [ ] Related Flarum core extension PR's: (Omit this section if irrelevant)