'tests setting a set to None creates an empty update statement'

def test_null_update(self):

    c = SetUpdateClause('s', None, previous=set((1, 2)))
    c._analyze()
    c.set_context_id(0)
    self.assertIsNone(c._assignments)
    self.assertIsNone(c._additions)
    self.assertIsNone(c._removals)
    self.assertEqual(c.get_context_size(), 0)
    self.assertEqual(str(c), '')
    ctx = {}
    c.update_context(ctx)
    self.assertEqual(ctx, {})
