Avoid updating modified_by from None to None (#11838)

This should help the case of inventory updates in particular
  where imported hosts are managed by the system

awx/main/tests/functional/models/test_base.py

@@ -0,0 +1,40 @@
+from unittest import mock
+import pytest
+
+from crum import impersonate
+
+from awx.main.models import Host
+
+
+@pytest.mark.django_db
+def test_modified_by_not_changed(inventory):
+    with impersonate(None):
+        host = Host.objects.create(name='foo', inventory=inventory)
+        assert host.modified_by == None
+        host.variables = {'foo': 'bar'}
+        with mock.patch('django.db.models.Model.save') as save_mock:
+            host.save(update_fields=['variables'])
+            save_mock.assert_called_once_with(update_fields=['variables'])
+
+
+@pytest.mark.django_db
+def test_modified_by_changed(inventory, alice):
+    with impersonate(None):
+        host = Host.objects.create(name='foo', inventory=inventory)
+        assert host.modified_by == None
+    with impersonate(alice):
+        host.variables = {'foo': 'bar'}
+        with mock.patch('django.db.models.Model.save') as save_mock:
+            host.save(update_fields=['variables'])
+            save_mock.assert_called_once_with(update_fields=['variables', 'modified_by'])
+        assert host.modified_by == alice
+
+
+@pytest.mark.django_db
+def test_created_by(inventory, alice):
+    with impersonate(alice):
+        host = Host.objects.create(name='foo', inventory=inventory)
+        assert host.created_by == alice
+    with impersonate(None):
+        host = Host.objects.create(name='bar', inventory=inventory)
+        assert host.created_by == None