Make LoadStaging.nodegroup nullable

[jacobtylerwalls]

#10885 has a use case for writing LoadStaging records associated with error conditions more generic than any specific nodegroup.

For instance, the JSON-LD parser might have been unable to match a node at all:

arches/arches/app/utils/data_management/resources/formats/rdffile.py

Line 688 in 43122e2

raise ValueError(f"Data does not match any actual node, despite prop/class combination {k} {clss}:\n{vi}")

or may have failed very early when validating ontologies:

arches/arches/app/utils/data_management/resources/formats/rdffile.py

Lines 410 to 412 in 43122e2

	# Possibly bail very early
	if jsonld_document["@type"][0] != self.graphtree["class"]:
	raise ValueError("Instance does not have same top level class as model")

In both of these cases, I'm getting around the fact that LoadStaging.nodegroup is not nullable by inserting a random nodegroup, which isn't ideal:

arches/arches/app/etl_modules/jsonld_importer.py

Lines 36 to 38 in ed4dbd8

	def fallback_nodegroup_id():
	"""Consider removing this if we make LoadStaging.nodegroup nullable."""
	return Node.objects.filter(nodegroup__isnull=False).first().nodegroup_id

Then, given that tile nodegroups will be nullable in v8, we probably have an additional use case for this.

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Suggesting to make LoadStaging.nodegroup nullable and then make sure the db functions account for this when doing their joins to produce error report tables.