potential bug: possible to create dangling landscape without associated group

[shrouxm]

Description

If this condition in the landscape creation code fails:

terraso-backend/terraso_backend/apps/core/models/landscapes.py

Line 107 in 560a3d9

if creating and self.created_by:

then there will be a landscape with no associated group in the DB. I think with the current code logic this is not possible, but we could silently open ourselves up to this bug in the future if the logic changes such that self.created_by is not always non-null. This should probably throw an error so that the landscape does not get created in this case. But I am not sure about this.

Additional context

I don't think this is currently a problem, but just wanted to record this observation somewhere. Could be a good first issue to investigate further.