Forbid optional Primary Keys

[sync-by-unito]

currently, core allows for nullable primary keys which is a divergence from how mobile developers normally think about primary keys. Generally, primary keys are assumed to be implicitly required, SQLite has this requirement, along with just about every other database ever. I am not sure what the reasoning was behind allowing this in Core but we should aim to align with mobile developers expectations 

This artifact has created downstream effects in Sync

[cmelchior]

The reason being is primarily that we are trying to expose our supported types as native types, and in many languages, these are nullable, like String in Java.

We can turn it non-nullable by a @Required annotation, but nothing in the type system will prevent you from doing the wrong thing and assign it null, which wouldn't be caught until at runtime.

[sync-by-unito]

➤ Ian Ward commented:

So MongoDB allows nullable primary keys and I guess we should align with that.