Add a missing test case for the persistence behavior of `serialize`

`serialize` makes the contract that if it is given a class name, it will
never return something other than an instance of that class. This means
that it must cast `nil` to the empty form of that object. As such, we
should then persist empty forms of that object as `nil`. While this is
techincally under the contract of

```
model.attribute = value
assert_equal model.attribute, model.tap(&:save).reload.attribute
```

which we can't actually test universally without property based testing,
it has come up more than once and is worth calling out specifically
since we aren't looking to change it.

0 files changed, 0 insertions, 0 deletions