Skip writing undefined fields and warn in dev

[kitten]

We hit a case where an undefined value was written to the cache which lead to an incomplete query. This will most likely occur with invalid updateQuery calls or invalid optimistic results that include undefined fields, where the query expects a value.

We now skip writing these fields and instead warn about them. It should never be acceptable to write undefined to the cache, as we only want to write null. In rare cases undefined will indeed be written to remove records, but that's done with the explicit store.removeRecord call.

We're indeed seeing some warnings for this case in the write.test.ts tests, but I'm pretty sure those were expected before.

[JoviDeCroock]

Let's add a test for this case, this will avoid us encountering this bug in the future