Fix error in `BlossomUser.date_last_active` calculation

[TimJentzsch]

blossom/blossom/authentication/models.py

Lines 116 to 128 in 350eb08

	recently_claimed = (
	Submission.objects.filter(claimed_by=self).order_by("-claim_time").first()
	)
	recent_claim_time = recently_claimed.claim_time if recently_claimed else None
	recently_completed = (
	Submission.objects.filter(completed_by=self)
	.order_by("-complete_time")
	.first()
	)
	recent_complete_time = (
	recently_completed.complete_time if recently_completed else None
	)

When there are null entries for complete_time or claim_time, these actually appear first with the -complete_time and -claim_time orderings.

So we need to explicitly filter out null entries, e.g. by using complete_time__isnull=False.