Account for null XForm instances in service_usage endpoint

[LMNTL]

Checklist

1. If you've added code that should be tested, add tests
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Description

Fix /api/v2/service_usage/ calculating NLP and submission counts incorrectly.

Notes

Deleting a form with submissions currently causes the submission counter to decrement. The endpoint previously did handle this correctly, but it was refactored as part of feature/usage-limits, which introduced this regression. See kobocat#895 for additional background.

Related issues

Depends on kobotoolbox/kobocat#895

See issue on Release 2.023.37 QA board.

[noliveleger]

user_assets seems to be used no where else than to be used to init asset_list
I would probably get rid of user_assets and assign directly asset_list with the results of queryset with values_list()

Moreover, the query could simplified with the new date_deployed field. Only assets of type 'survey' are deployed. We just need to check whether the value of the field is not null to get all deployed projects.

So what about this?

        asset_list = list(
            Asset.all_objects.values_list('uid', flat=True).filter(
                owner__in=self._users,
                date_deployed__isnull=False,
            )
        )
        
        xforms = KobocatXForm.objects.only('bytes_sum', 'id', 'kpi_asset_uid').filter(
            kpi_asset_uid__in=asset_list,
        )