When the related model uses SoftDeletes, `isRelationNullable()` was
forcing the BelongsTo relation type to nullable even if the relation
explicitly opted into trashed parents via `->withTrashed()`. Combined
with a NOT NULL FK column and DB-level FK constraint, the relation is
effectively non-nullable.
The SoftDeletes branch is now skipped when the relation has removed
the `SoftDeletingScope` and added no constraint on the qualified
`deleted_at` column. `->onlyTrashed()` and `->withoutTrashed()` keep
their nullable annotation because they restrict the parent to a
specific soft-delete state.
Co-authored-by: Claude Opus 4.7 (1M context) <[email protected]>
