fix(ndb): add test case for _properties is None in StructuredProperty._set_projection

Author: ohmayr

packages/google-cloud-ndb/tests/unit/test_model.py

@@ -6670,3 +6670,5 @@ class Parent(model.Model):
 
     parent = Parent(child=Child(val="a"), children=[Child(val="b")])
     parent._set_projection(["child.val", "children.val", "nonexistent.subprop"])
+    parent._properties = None
+    parent._set_projection(["child.val"])