fix: serialize None to null, for ObjectRef schema type

not sure how this didn't come up until now..anyway hopefully it's the
right thing but we'll see

src/wuttaweb/forms/schema.py

@@ -357,15 +357,17 @@ class ObjectRef(colander.SchemaType):
 
     def serialize(self, node, appstruct):  # pylint: disable=empty-docstring
         """ """
-        # nb. normalize to empty option if no object ref, so that
-        # works as expected
+        # normalize to empty option if no object ref, so that works as
+        # expected
         if self.empty_option and not appstruct:
             return self.empty_option[0]
 
-        if appstruct is colander.null:
+        # even if there is no empty option, still treat any false-ish
+        # value as null
+        if not appstruct:
             return colander.null
 
-        # nb. keep a ref to this for later use
+        # keep a ref to this for later use
         node.model_instance = appstruct
 
         # serialize to PK as string

tests/forms/test_schema.py

@@ -257,6 +257,8 @@ class TestObjectRef(DataTestCase):
         typ = mod.ObjectRef(self.request)
         value = typ.serialize(node, colander.null)
         self.assertIs(value, colander.null)
+        value = typ.serialize(node, None)
+        self.assertIs(value, colander.null)
 
         # model instance
         person = model.Person(full_name="Betty Boop")