Missing nodes in __get_nodegroup_tree

[jacobtylerwalls]

The db functions __get_nodegroup_tree or __get_nodegroup_tree_by_graph appear to miss child nodes, unless I'm misunderstanding how they work.

With this setup:

from pathlib import Path

from django.conf import settings
from django.core.management import call_command

from arches.app.utils.betterJSONSerializer import JSONDeserializer
from arches.app.utils.data_management.resource_graphs.importer import import_graph as ResourceGraphImporter
from arches.app.utils.skos import SKOSReader

ROOT_DIR = settings.ROOT_DIR
JSON_LD_FIXTURE = Path(ROOT_DIR).parent / "tests" / "fixtures" / "jsonld_base"
ONTOLOGY_PATH = Path(ROOT_DIR).parent / "tests" / "fixtures" / "ontologies" / "cidoc_crm"
call_command("load_ontology", source=ONTOLOGY_PATH, verbosity=0)

skos = SKOSReader()
rdf = skos.read_file(str(JSON_LD_FIXTURE / "rdm" / "jsonld_test_thesaurus.xml"))
ret = skos.save_concepts_from_skos(rdf)

skos = SKOSReader()
rdf = skos.read_file(str(JSON_LD_FIXTURE / "rdm" / "jsonld_test_collections.xml"))
ret = skos.save_concepts_from_skos(rdf)

with open(JSON_LD_FIXTURE / "models" / "test_2_complex_object.json", "r") as f:
    archesfile2 = JSONDeserializer().deserialize(f)

ResourceGraphImporter(archesfile2['graph'])

SELECT
	COUNT(nodegroupid)
FROM
	nodes
WHERE
	graphid = 'ee72fb1e-fa6c-11e9-b369-3af9d3b32b71';

-- Result: 13

SELECT
	COUNT(nodegroupid)
FROM
	__get_nodegroup_tree_by_graph ('ee72fb1e-fa6c-11e9-b369-3af9d3b32b71');

-- Result: 9, expected 12 (seems okay to omit the top node)

-- Next query shows what's missing: just the child nodes
SELECT
	*
FROM
	nodes
WHERE
	graphid = UUID('ee72fb1e-fa6c-11e9-b369-3af9d3b32b71')
	AND alias NOT IN(
		SELECT
			alias FROM __get_nodegroup_tree_by_graph (UUID('ee72fb1e-fa6c-11e9-b369-3af9d3b32b71')));

---

Noticed while dev'ing #10798